Net, libertas: Resolve memory leak in if_spi_host_to_card()

If we hit the default case in the switch in if_spi_host_to_card() we'll leak
the memory we allocated for 'packet'. This patch resolves the leak by freeing
the allocated memory in that case.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Dan Williams <dcbw@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>