spi: bcm63xx-hsspi: checking for ERR_PTR instead of NULL

devm_request_and_ioremap() returns NULL on error, it doesn't return an
ERR_PTR().  This patch fixes it by switching to devm_ioremap_resource()
which is the prefered function anyway.

Fixes: 142168eb ('spi: bcm63xx-hsspi: add bcm63xx HSSPI driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Mark Brown <broonie@linaro.org>