scsi: scsi_transport_spi: Fix function pointer check

clang static analysis flags several null function pointer problems.

drivers/scsi/scsi_transport_spi.c:374:1: warning: Called function pointer is null (null dereference) [core.CallAndMessage]
spi_transport_max_attr(offset, "%d\n");
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reviewing the store_spi_store_max macro

	if (i->f->set_##field)
		return -EINVAL;

should be

	if (!i->f->set_##field)
		return -EINVAL;

Link: https://lore.kernel.org/r/20200627133242.21618-1-trix@redhat.com
Reviewed-by: James Bottomley <jejb@linux.ibm.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>