NOR/CFI: use bus_width for memory access in cfi_write()

NOR/CFI: use bus_width for memory access in cfi_write()

During cfi_write(), head and tail of destination area
could be not aligned to bus_width.
Since write operation must be at bus_width size, source
buffer size is extended and buffer padded with current
values read from flash.

Force using bus_width to read current value from flash.
Do not use cfi_add_byte() anymore, to allow removing this
function later on.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>