xhci-mem.c: xhci_segment_free: No need for checking seg argument

The seg argument to xhci_segment_free is never passed as NULL, so
no need to check for this in xhci_segment_free.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>