Use pwrite_full() instead of pwrite() to make sure that all of the
data was written to target device. While at it adjust return value of
imx_bbu_write_device() to be zero in case of success.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>