spi: spi-mem: Fix passing zero to 'PTR_ERR' warning

Fix smatch warning:

drivers/spi/spi-mem.c:746 spi_mem_probe() warn: passing zero to 'PTR_ERR'

Fixes: 5d27a9c8 ("spi: spi-mem: Extend the SPI mem interface to set a custom memory name")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20201031033042.42892-1-yuehaibing@huawei.com


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