The error check of mbr < 0 is always false because mbr is a u32. Make
mbt an int so that a -ve error return from stm32_spi_prepare_mbr can be
detected.

Detected by CoverityScan, CID#1446586 ("Unsigned compared against 0")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NMark Brown <broonie@kernel.org>