obexd/client: Fix calling g_obex_cancel_req for completed requests

obexd/client: Fix calling g_obex_cancel_req for completed requests

This leads to incorrect responses such as the following:

> ACL data: handle 12 flags 0x02 dlen 26
    L2CAP(d): cid 0x0041 len 22 [psm 3]
      RFCOMM(d): UIH: cr 0 dlci 38 pf 0 ilen 18 fcs 0xfd
        OBEX: Get rsp(f): status 200 len 18
        Status 200 = Success
        Connection ID (0xcb) = 1
        App. Parameters (0x4c) = Sequence length 4
        End of Body (0x49) = Sequence length 0
< ACL data: handle 12 flags 0x00 dlen 11
    L2CAP(d): cid 0x0040 len 7 [psm 3]
      RFCOMM(d): UIH: cr 1 dlci 38 pf 0 ilen 3 fcs 0x27
        OBEX: Get rsp(f): status 715 len 3