bluez5: Fix ptest hang

bluez5: Fix ptest hang

The below test hangs infinitely
$ ./run-ptest

Actually it stuck at test-gatt unit test and
the detailed test output as below:
$ unit/test-gatt -p  /robustness/unkown-request -d
/robustness/unkown-request - init
/robustness/unkown-request - setup
/robustness/unkown-request - setup complete
/robustness/unkown-request - run
GATT: < 02 17 00                                         ...
bt_gatt_server:MTU exchange complete, with MTU: 23
GATT: > 03 00 02                                         ...
PDU: = 03 00 02                                         ...
GATT: < bf 00

Considering the /robustness/unkown-request test
actually does no action, update to raw_pdu() to
fix the hang issue.

Signed-off-by: Mingli Yu <Mingli.Yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>