mtd: spi-nor: cadence-quadspi: fix timeout handling

wait_for_completion_timeout returns an unsigned long not an int, so
let's check its return value directly instead of storing it in ret,
and avoid checking for negative values since this cannot happen.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>