Fix a static code checker warning:
drivers/spi/spi-bcm2835aux.c:460
 bcm2835aux_spi_probe() warn: passing zero to 'PTR_ERR'

In case of error, the function devm_clk_get() returns ERR_PTR()
and not returns NULL.
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NMark Brown <broonie@kernel.org>