libertas : Remove unused variable warning for "old_channel" from cmd.c

Below patch removes the following warning during compilation.

drivers/net/wireless/libertas/cmd.c:826: warning: unused variable 'old_channel'

Signed-off-by : Manish Katiyar <mkatiyar@gmail.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>