linux-user: Simplify timerid checks on g_posix_timers range

We check whether the passed in timer id is negative on all calls
that involve g_posix_timers.

However, these checks are bogus. First off we limit the timer_id to
16 bits which is not what Linux does. Then we check whether it's negative
which it can't be because we masked it.

We can safely remove the masking. For the negativity check we can just
treat the timerid as unsigned and only check for upper boundaries.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
