greybus: loopback: fix the type attribute check

In gb_loopback_check_attr(), the value of gb->type is checked for
validity.  The only valid values are 0, 1, and 2.  But the check
allows the value 3.  Fix that.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>