staging: greybus: uart: fix TIOCGSERIAL flags

This driver does not have a low-latency mode and should not report
anything else.

Also drop the skip-test flag which isn't used either.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>