drivers/onewire: Fix undefined variable errors.
drivers/onewire: Fix undefined variable errors.

On CPython, and with pylint, the variables MATCH_ROM and SEARCH_ROM are
undefined.  This code works in MicroPython because these variables are
constants and the MicroPython parser/compiler optimises them out.  But it
is not valid Python because they are technically undefined within the scope
they are used.

This commit makes the code valid Python code.  The const part is removed
completely because these constants are part of the public API and so cannot
be moved to the global scope (where they could still use the MicroPython
const optimisation).

kerneltask/micropython,pramasoul/micropython,bvernoux/micropython,kerneltask/micropython,pramasoul/micropython,bvernoux/micropython,selste/micropython,MrSurly/micropython,kerneltask/micropython,pramasoul/micropython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython,MrSurly/micropython,bvernoux/micropython,tobbad/micropython,selste/micropython,adafruit/circuitpython,tobbad/micropython,adafruit/circuitpython,pozetroninc/micropython,kerneltask/micropython,pozetroninc/micropython,tobbad/micropython,selste/micropython,pozetroninc/micropython,bvernoux/micropython,adafruit/circuitpython,tobbad/micropython,pozetroninc/micropython,pramasoul/micropython,pramasoul/micropython,pozetroninc/micropython,adafruit/circuitpython,selste/micropython,henriknelson/micropython,tobbad/micropython,MrSurly/micropython,henriknelson/micropython,MrSurly/micropython,adafruit/circuitpython,henriknelson/micropython,kerneltask/micropython,selste/micropython,henriknelson/micropython,MrSurly/micropython