libertas: remove redundant assignment to variable ret

The variable ret is being assigned a value that is never read and is
being re-assigned a little later on. The assignment is redundant and hence
can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>