On OS X, `netdev2_tap` suspends monitoring file descriptor until `_recv` is
called. If no spaces in left in pktbuf, `gnrc_netdev2_eth` does not call `_recv`
that results in deadlock.
With this commit, `gnrc_netdev2_eth` calls `_recv` with NULL buffer and non-zero
length parameter, that indicates the driver to drop frame and resume working.