Always check for no GATT transaction before sending request

After completing a GATT request in cl_op_cmpl(), we invoke a callback
and send the next request. However, the callback mentioned can
potentially send another request via accept_client_operation(). Since
we shouldn't send two simultaneous request in GATT, this might end up
being problematic, such as mismatching the response.

This CL makes sure there are no parallel running request before
sending the next request.

Bug: 279881847
Test: Manual pairing/poweroff/poweron/forget loop with MX Master 2S
      mouse. Verify the device name never gets overwritten with
      wrong values.
Change-Id: I08cca1dd588f748d253d6007ad50ac10d62668d4