cfi: fix the incomplete erased status check in buffer write

Without the fix, flash_write_cfibuffer will terminate the erased
status check once an all-0xFF word has been found instead of
continuing the erased status check utill the first non-0xFF word.

Signed-off-by: Tao Hou <hotforest@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>