wifi: rtl8xxxu: Support new chip RTL8710BU aka RTL8188GU

This chip is found in cheap "free driver" USB adapters from Aliexpress.
Initially they pretend to be a CD-ROM containing the driver for Windows.
"Ejecting" switches the device to wifi mode.

Features: 2.4 GHz, b/g/n mode, 1T1R, 150 Mbps.

This chip is more unique than other Realtek chips:

* The registers at addresses 0x0-0xff, which all the other chips use,
  can't be used here. New registers at 0x8000-0x80ff must be used
  instead. And it's not a simple matter of adding 0x8000: 0x2
  (REG_SYS_FUNC) became 0x8004, 0x80 (REG_MCU_FW_DL) became 0x8090,
  etc.

* Also there are a few new registers which must be accessed indirectly
  because their addresses don't fit in 16 bits. No other chips seem to
  have these.

* The vendor driver compiles to 8188gu.ko, but the code calls the chip
  RTL8710B(U) pretty much everywhere, including messages visible to the
  user.

Another difference compared to the other chips supported by rtl8xxxu is
that it has a new PHY status struct, or three of them actually, from
which we extract the RSSI, among other things. This is not unique,
though, just new. The chips supported by rtw88 also use it.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/4edbe29f-00b9-8eef-9789-20bed0b141e2@gmail.com