wifi: rtw89: cleanup firmware elements parsing

When compiling with clang-18, I've noticed the following:

drivers/net/wireless/realtek/rtw89/fw.c:389:28: warning: cast to smaller
integer type 'enum rtw89_fw_type' from 'const void *' [-Wvoid-pointer-to-enum-cast]
  389 |         enum rtw89_fw_type type = (enum rtw89_fw_type)data;
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/realtek/rtw89/fw.c:569:13: warning: cast to smaller
integer type 'enum rtw89_rf_path' from 'const void *' [-Wvoid-pointer-to-enum-cast]
  569 |                 rf_path = (enum rtw89_rf_path)data;
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~

So avoid brutal everything-to-const-void-and-back casts, introduce
'union rtw89_fw_element_arg' to pass parameters to element handler
callbacks, and adjust all of the related bits accordingly. Compile
tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231020040940.33154-1-dmantipov@yandex.ru