Bluetooth: Check earlier for L2CAP ERTM frames to drop

Even when the received tx_seq is expected, the frame still needs to be
dropped if the TX window is exceeded or the receiver is in the local
busy state.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>