media: mediatek: vcodec: avoid -Wcast-function-type-strict warning

The ipi handler here tries hard to maintain const-ness of its argument,
but by doing that causes a warning about function type casts:

drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_fw_vpu.c:38:32: error: cast from 'mtk_vcodec_ipi_handler' (aka 'void (*)(void *, unsigned int, void *)') to 'ipi_handler_t' (aka 'void (*)(const void *, unsigned int, void *)') converts to incompatible function type [-Werror,-Wcast-function-type-strict]
   38 |         ipi_handler_t handler_const = (ipi_handler_t)handler;
      |                                       ^~~~~~~~~~~~~~~~~~~~~~

Remove the hack and just use a non-const argument.

Fixes: bf1d556a

 ("media: mtk-vcodec: abstract firmware interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>