spi: spi-fsl-qspi: Fix return value check of devm_ioremap() in probe

In case of error, the function devm_ioremap() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: 858e26a5

 ("spi: spi-fsl-qspi: Reduce devm_ioremap size to 4 times AHB buffer size")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Ashish Kumar <Ashish.Kumar@nxp.com>
Acked-by: Han Xu <han.xu@nxp.com>
Link: https://lore.kernel.org/r/20200422014543.111070-1-weiyongjun1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>