spi: ti-qspi: Make sure res_mmap != NULL before dereferencing it

resource_size() is dereferencing the res without checking that it is
not NULL, so we need to do the check before calling resource_size().

Fixes: b95cb394 ("spi: ti-qspi: Implement the spi_mem interface")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>