coresight: stm: Remove redundant NULL checks

container_of() cannot return NULL, so the checks for NULL pointers are
unnecessary and can be safely removed.

As a result, this commit silences the following smatch warnings:

  coresight-stm.c:345 stm_generic_link() warn: can 'drvdata' even be NULL?
  coresight-stm.c:356 stm_generic_unlink() warn: can 'drvdata' even be NULL?
  coresight-stm.c:387 stm_generic_set_options() warn: can 'drvdata' even be NULL?
  coresight-stm.c:422 stm_generic_packet() warn: can 'drvdata' even be NULL?

Signed-off-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20250611-arm_cs_fix_smatch_warning_v1-v1-1-02a66c69b604@arm.com