rtw88: Use rtw_write8_set to set SYS_FUNC

rtw_write8 could modify the values that we do not want
to change, use rtw_write8_set instead to only enable the
bits of sys_func_en

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>