msm: emac: Fix memory leak when receiving jumbo packets

The driver uses the null pointer to free the skb buffer when handling
the RX error condition. It causes the memory leak.

Change-Id: I5971e920aa5e6865598c6028dd87f4213c5d5b41
Acked-by: Jie Luo <jluo@qti.qualcomm.com>
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>