spi: spi-mem: Fix build error without CONFIG_SPI_MEM

When building with CONFIG_SPI_MEM is not set
gc warns this:

drivers/spi/spi-zynq-qspi.o: In function `zynq_qspi_supports_op':
spi-zynq-qspi.c:(.text+0x1da): undefined reference to `spi_mem_default_supports_op'

Fixes: 67dca5e5 ("spi: spi-mem: Add support for Zynq QSPI controller")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>