flash: Fix CFI buffer size bug

Fix bug introduced by 9c048b52.

The cfi_flash.c driver cast the flash buffer size to a uchar in
flash_write_cfibuffer(). On some flash parts, (tested on Numonyx
part PC32F512M29EWH), the buffer size is 1KB. Remove the cast to
uchar to enable buffer sizes to be larger.

Signed-off-by: John Schmoller <jschmoller@xes-inc.com>
Signed-off-by: Stefan Roese <sr@denx.de>