applicom: fix some err codes returned by ac_ioctl

When cmd > 6 or copy_to_user() fail, The variable 'ret' would not be
returned back. Fix the 'ret' set but not used.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Xu Jia <xujia39@huawei.com>
Link: https://lore.kernel.org/r/20210324072031.941791-1-xujia39@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>