mtd: pmc551: fix signedness bug in init_pmc551()

Since "length" is a u32, the error handling below didn't work when
fixup_pmc551() returns -ENODEV.

	if ((length = fixup_pmc551(PCI_Device)) <= 0)

This patch changes both the type of "length" and the return type of
fixup_pmc551() to int.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>