drivers/memory/spiflash: Fix bugs in and clean up read/write functions.
drivers/memory/spiflash: Fix bugs in and clean up read/write functions.

mp_spiflash_read had a bug in it where "dest" and "addr" were incremented
twice for a certain special case.  This was fixed, which then allowed the
function to be simplified to reduce code size.

mp_spiflash_write had a bug in it where "src" was not incremented correctly
for the case where the data to be written included the caching buffer as
well as some bytes after this buffer.  This was fixed and the resulting
code simplified.

selste/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/micropython,tobbad/micropython,pramasoul/micropython,bvernoux/micropython,swegener/micropython,adafruit/micropython,kerneltask/micropython,tobbad/micropython,kerneltask/micropython,adafruit/micropython,MrSurly/micropython,bvernoux/micropython,adafruit/circuitpython,swegener/micropython,dmazzella/micropython,pramasoul/micropython,selste/micropython,adafruit/circuitpython,trezor/micropython,kerneltask/micropython,ryannathans/micropython,dmazzella/micropython,pozetroninc/micropython,trezor/micropython,kerneltask/micropython,pozetroninc/micropython,MrSurly/micropython,pfalcon/micropython,pozetroninc/micropython,pramasoul/micropython,adafruit/circuitpython,MrSurly/micropython,pozetroninc/micropython,bvernoux/micropython,swegener/micropython,henriknelson/micropython,bvernoux/micropython,tralamazza/micropython,tobbad/micropython,tobbad/micropython,henriknelson/micropython,MrSurly/micropython,ryannathans/micropython,henriknelson/micropython,ryannathans/micropython,tobbad/micropython,tralamazza/micropython,dmazzella/micropython,swegener/micropython,swegener/micropython,adafruit/micropython,ryannathans/micropython,trezor/micropython,adafruit/micropython,pramasoul/micropython,henriknelson/micropython,selste/micropython,pfalcon/micropython,trezor/micropython,selste/micropython,trezor/micropython,selste/micropython,adafruit/circuitpython,pramasoul/micropython,tralamazza/micropython,ryannathans/micropython,dmazzella/micropython,pozetroninc/micropython,tralamazza/micropython,pfalcon/micropython,kerneltask/micropython,pfalcon/micropython,henriknelson/micropython,MrSurly/micropython,adafruit/circuitpython,pfalcon/micropython