rtlwifi: rf_lock use non-irqsave spin_lock

rf_lock is used to protect RF register access, but they will not called
from interrupt context, so *_irqsave version isn't necessary. Then, these
delays don't affect IRQ services.

The old code holds spin_lock_irqsave() that will be detected a long delay
as below:

  kworker/-276     4d...    0us : _raw_spin_lock_irqsave
  kworker/-276     4d...    0us : rtl8723_phy_rf_serial_read <-rtl8723de_phy_set_rf_reg
  kworker/-276     4d...    1us : rtl8723_phy_query_bb_reg <-rtl8723_phy_rf_serial_read
  kworker/-276     4d...    3us : rtl8723_phy_set_bb_reg <-rtl8723_phy_rf_serial_read
  kworker/-276     4d...    4us : __const_udelay <-rtl8723_phy_rf_serial_read
  kworker/-276     4d...    4us!: delay_mwaitx <-rtl8723_phy_rf_serial_read
  kworker/-276     4d... 1004us : rtl8723_phy_set_bb_reg <-rtl8723_phy_rf_serial_read
  [...]

Reported-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>