spi: bcm-qspi: Fix return value check in bcm_qspi_probe()

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

Fixes: fa236a7e ("spi: bcm-qspi: Add Broadcom MSPI driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>