mtd: mxc-nand: Do the word to byte mangling in the read_byte callback

When the hardware operates in 16 bit mode it always reads 16 bits even
for operations that only have the lower 8 bits defined. So the upper
bits must be discarded. Do this in the read_byte callback instead of
when reading the NAND id to support reading byte wise more than 5 bytes
and at other occations (like reading the ONFI parameter page).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>