clocksource/drivers/imx: Fix -Wunused-but-set-variable warning

clocksource/drivers/imx: Fix -Wunused-but-set-variable warning

All warnings (new ones prefixed by >>):

   drivers/clocksource/timer-imx-gpt.c: In function 'mxc_timer_interrupt':
>> drivers/clocksource/timer-imx-gpt.c:279:18: warning: variable 'tstat' set but not used [-Wunused-but-set-variable]
     279 |         uint32_t tstat;
         |                  ^~~~~

vim +/tstat +279 drivers/clocksource/timer-imx-gpt.c

The change remove the tstats assignment but not the reading of the
register, assuming the register may be a ROR (Reset On Read) which
happens in the driver's interrupt registers.

Fixes: df181e382816 ("clocksource/drivers/imx-gpt: Add support for ARM64")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312231803.XzPddRa5-lkp@intel.com/
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231227143546.2823683-1-daniel.lezcano@linaro.org