staging: gdm72xx: return -EINVAL instead of BUG_ON for invalid data length

This patch changes gdm_usb_send() and gdm_sdio_send() to return -EINVAL instead
of calling BUG_ON if an invalid data length is passed to the functions.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Michalis Pappas <mpappas@fastmail.fm>
Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>