Bluetooth: Fix using uninitialized variable

+ src/net/bluetooth/rfcomm/tty.c: warning: 'p' is used uninitialized in this
       function:  => 218
+ src/net/bluetooth/rfcomm/tty.c: warning: 'p' may be used uninitialized in
       this function:  => 218

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>