gatt: Fix possible MTU Request stuck in the bta queue

Following MTU Request should not be enque for exectution but simply put
in the queue, which later is checked by the code when MTU Exchange
procedure is ended.

Bug: 311830877
Test: manual stress testing of LE HS reconnections.
Tag: Feature
Change-Id: Ic90cadbebdcc52a496f2b1eea4e9a18746e5b327