usb-storage: Drop useless null test in usb_msd_handle_data()

scsi_req_new() never returns null, and scsi_req_enqueue() dereferences
the pointer, so checking for null is useless.

Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
