Bluetooth: hci_ll: Fix firmware leak on error path

Smatch reports:

drivers/bluetooth/hci_ll.c:587 download_firmware() warn:
'fw' from request_firmware() not released on lines: 544.

In download_firmware(), if request_firmware() succeeds but the returned
firmware content is invalid (no data or zero size), the function returns
without releasing the firmware, resulting in a resource leak.

Fix this by calling release_firmware() before returning when
request_firmware() succeeded but the firmware content is invalid.

Fixes: 371805522f87 ("bluetooth: hci_uart: add LL protocol serdev driver support")
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Anas Iqbal <mohd.abd.6602@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>