Bluetooth: AT: Fix - chance of missing UNSOLICITED state

If during hfp_hf_send response callback is not filled with NULL
and you receive unsolicited callback. which will not be taken care.
This patch fixes this issue.

Change-Id: I04007059d62273b9cdddf29e2d4a9086b07a01e5
Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>