rtw88: Use rtw_write8_set to set SYS_FUNC

rtw88: Use rtw_write8_set to set SYS_FUNC

BugLink: https://bugs.launchpad.net/bugs/1780590
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>
(cherry picked from commit a4835410995e0a15b40cd4ab13d19e680f5a2602)
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>