wifi: rtl8xxxu: 8188f: Limit TX power index

TX power index is read from the efuse on init, the values get written to
the TX power registers when the channel gets switched.

When the chip has not yet been calibrated, the efuse values are 0xFF,
which on some boards leads to USB timeouts for reading/writing registers
after the first frames have been sent.

The vendor driver (v5.11.5-1) checks for these invalid values and sets
default values instead. Implement something similar in
rtl8188fu_parse_efuse().

Fixes: c888183b ("wifi: rtl8xxxu: Support new chip RTL8188FU")
Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240624140037.231657-1-martin.kaistra@linutronix.de