ath9k: fix a memory leak in ath_rx_tasklet()

commit 0d95521e (ath9k: use split rx buffers to get rid of order-1 skb
allocations) added in memory leak in error path.

sc->rx.frag should be cleared after the pskb_expand_head() call, or else
we jump to requeue_drop_frag and leak an skb.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jouni Malinen <jouni@qca.qualcomm.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: John W. Linville <linville@tuxdriver.com>
Cc: Trond Wuellner <trond@chromium.org>
Cc: Grant Grundler <grundler@chromium.org>
Cc: Paul Stewart <pstew@chromium.org>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>