Bluetooth: L2CAP: Fix unnecessary NULL check

The segment allocation function can't fail (it eventually waits with
K_FOREVER for a buffer to become available), so there's no point in
checking its return value for NULL. Also, the connection state check
is because of this particular waiting and not the semaphore waiting
(which is done with K_NO_WAIT).

Change-Id: I9698760541de810869cffc1c60cf97c5f8f7df8d
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>