gatt: Fix possible assert on GATT timeout while ACL disconnection

This solves race when gatt is handling timeout on ATT which was sent on
EATT, but EATT channel is not existing anymore.

Bug: 284908879
Test: atest BluetoothInstrumentationTests
Tag: #feature
Change-Id: I1aa9d859269df8853d6ba03a964200625a1581ac