IB/usnic: Fix incorrect cast in usnic_ib_fw_string_to_u64

Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Nelson Escobar <neescoba@cisco.com>
Reviewed-by: Dave Goodell <dgoodell@cisco.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>