sparc: Replace test_ti_thread_flag() with test_tsk_thread_flag()

Use test_tsk_thread_flag() directly instead of test_ti_thread_flag() to
improve readability when the argument type is struct task_struct, it is
similar with commit 5afc7855 ("arm64: Use test_tsk_thread_flag() for
checking TIF_SINGLESTEP").

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>