header.minor is of type u8 and cannot be negative.

Detected by CoverityScan CID#1417858 ("Integer handling issues")

Fixes: f384b352 ("mtd: spi-nor: parse Serial Flash Discoverable
Parameters (SFDP) tables")
Signed-off-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NCyrille Pitchen <cyrille.pitchen@wedev4u.fr>