mtd: spi-nor: cqspi: remove redundant dead code on error return check

Checking for ret < 0 is redundant because a previous check on ret
being non-zero already handles the ret < 0 case. Remove the redundant
code. Found by CoverityScan, CID#1398863, CID#1398864

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>