mtd: spi-nor: Kill check with no effect

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: Richard Weinberger <richard@nod.at>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>