Bluetooth: Fix warning: using int as NULL pointer

Fix for warnings below:

...
drivers/bluetooth/bt3c_cs.c:667:20: warning: Using plain integer
	as NULL pointer
drivers/bluetooth/btuart_cs.c:596:20: warning: Using plain integer
	as NULL pointer
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>