drivers: bluetooth: h4: Free RX buffer upon reset

There are cases where reset_rx() is called when rx.buf is set (e.g. when
the buffer was too small to receive the incoming packet). Be sure to free
the buffer, so that it wont get reused for the next packet (which might
require a buffer from a different pool).

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>