coresight: stm: Remove set but not used variable 'res_size'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/hwtracing/coresight/coresight-stm.c: In function 'stm_probe':
drivers/hwtracing/coresight/coresight-stm.c:796:9: warning:
 variable 'res_size' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit 237483aa ("coresight: stm:
adding driver for CoreSight STM component")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>