Fix assertion failure in btif_sock_rfc

The BTIF RFCOMM code was ignoring the status code on RFCOMM writes. In
the failure case, the slot id is 0 which is invalid. But the slot id
shouldn't even be referenced since the result was failure.

Stack trace:

__assert2+20               bionic/libc/bionic/assert.cpp:40
find_rfc_slot_by_id+18     system/bt/main/../btif/src/btif_sock_rfc.c:144
on_rfc_write_done          system/bt/main/../btif/src/btif_sock_rfc.c:537
rfcomm_cback+632           system/bt/main/../btif/src/btif_sock_rfc.c:585
bta_jv_rfcomm_write+74     system/bt/bta/./jv/bta_jv_act.c:2121
bta_jv_sm_execute+18       system/bt/bta/./jv/bta_jv_main.c:93
bta_sys_event+56           system/bt/bta/./sys/bta_sys_main.c:523
run_reactor+254            system/bt/osi/./src/reactor.c:278
run_thread+142             system/bt/osi/./src/thread.c:209
__pthread_start(void*)+30  bionic/libc/bionic/pthread_create.cpp:199
__start_thread+6           bionic/libc/bionic/clone.cpp:41

Bug: 27819511
Change-Id: Ifcd19fdfb214559f345567f04db8146b30037363