spi: spi-nxp-fspi: Fix a NULL vs IS_ERR() check in probe

The platform_get_resource_byname() function returns NULL on error, it
doesn't return error pointers.

Fixes: d166a735 ("spi: fspi: dynamically alloc AHB memory")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200312113154.GC20562@mwanda


Signed-off-by: Mark Brown <broonie@kernel.org>