Bluetooth : Handling case: close comes before rxthread

Handles the situation where userial_open takes time in
BT_VND_OP_USERIAL_OPEN opening and meanwhile the close
request comes.This way it will lead to crash in second
open since call flow will be like open-->close-->userial
Rx thread created.

CRs-Fixed: 673965
Change-Id: I1d3b4b04700a9d0e235702d96f60fc6780fff430