- Avoid possible use of uninitialized variable; noticed by jsg@

- Avoid possible use of uninitialized variable; noticed by jsg@
- Only set value in R92C_FAST_EDCA_CTRL register if the device is
  combo WiFi/Bluetooth; confirmed by Yan-Hsuan Chuan from Realtek
- There's no need to set the default value for the R92C_PIFS since
  default value is 0x1c; confirmed by Yan-Hsuan Chuan from Realtek
- Disable usb Rx aggregation mode for rtl8188cu/rtl8192cu;
  vendor drivers use DMA Rx aggregation mode for rtl8188cu/rtl8192cu/rtl8188eu
  and use USB Rx aggregation mode for later chipsets.

Tested by stsp and me.

ok stsp@