spi: fsl-spi: fix devm_ioremap_resource() error case

devm_ioremap_resource() doesn't return NULL but an ERR_PTR on error.

Reported-by: Jonas Gorsky <jogo@openwrt.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>