pinctrl: mediatek: use spin lock in mtk_rmw

pinctrl: mediatek: use spin lock in mtk_rmw

Commit 90e2774dc252 ("pinctrl: add lock in mtk_rmw function.") uses
mutex lock in mtk_rmw.  However the function is possible called from
atomic context.

For example call trace:
  mutex_lock+0x28/0x64
  mtk_rmw+0x38/0x80
  [snip]
  max98357a_daiops_trigger+0x8c/0x9c
  soc_pcm_trigger+0x5c/0x10c

The max98357a_daiops_trigger() could run in either atomic or non-atomic
context.  As a result, dmesg shows some similar messages: "BUG: sleeping
function called from invalid context at kernel/locking/mutex.c:254".

Uses spin lock in mtk_rmw instead.

Fixes: 90e2774dc252 ("pinctrl: add lock in mtk_rmw function.")
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20210419093449.3125704-1-tzungbi@google.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>