QSPI: change length to be in/out parameter
QSPI: change length to be in/out parameter

This provides a way to return how many bytes have been written/read (as status codes
are returned via func ret value)

c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,betzw/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,c1728p9/mbed-os,betzw/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,c1728p9/mbed-os,betzw/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed