hw/arm/stellaris: Correct handling of GPTM TAR register

hw/arm/stellaris: Correct handling of GPTM TAR register

We don't implement very much of the GPTM TAR register, and what we
do is wrong. The "are we in RT mode?" field is in s->config, not
s->control. Correct this, use LOG_UNIMP rather than hw_error()
for the cases we don't support, and avoid an unlabelled fallthrough
that makes Coverity complain.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>