mailbox: arm_mhuv2: Skip calling kfree() with invalid pointer

It is possible that 'data' passed to kfree() is set to a error value
instead of allocated space. Make sure it doesn't get called with invalid
pointer.

Fixes: 5a6338cc ("mailbox: arm_mhuv2: Add driver")
Cc: v5.11 <stable@vger.kernel.org> # v5.11
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>