NVMe: Fix a signedness bug in nvme_trans_modesel_get_mp

nvme_trans_modesel_get_mp() was defined with a unsigned return
type, but can return signed values.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>