flash_otp_write: fix writing to NAND in presence of partial reads

flash_otp_write: fix writing to NAND in presence of partial reads

When doing something like:

	{ printf "\xff"; printf "\xfe"; } | flash_otp_write -u /dev/mtd0 0

flash_otp_write might see only a single byte when reading from stdin for
the first tim. In this case (and without this patch) it pads to
$writesize with '\xff's and writes that out. In the next iteration it
reads the 2nd byte, pads and writes again. So the 2nd byte is written to
offset $writesize instead of 1.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>