net: gemini: fix memory leak

cppcheck report:
[drivers/net/ethernet/cortina/gemini.c:543]: (error) Memory leak: skb_tab

Signed-off-by: Igor Pylypiv <igor.pylypiv@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>