cfi_flash: support of long cmd in U-boot.

Some NOR flash chips needs support of commands with length grether than max
value size of uchar. For example all M18 family chips use 0x1ff command in
buffered write mode as value of program loops count.

Signed-off-by: Alexey Korolev <akorolev@infradead.org>
Signed-off-by: Vasiliy Leonenko <vasiliy.leonenko@mail.ru>