imx-bbu-nand-fcb: When writing firmware return new bad blocks
Positive return values of imx_bbu_write_firmware() so far indicate
the last block that has been written to. This value is unused, so
return values > 0 to indicate if there are new bad blocks. This
information can be used in the next step to know if the DBBT has
to be rewritten.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>