wifi: rtw89: fix EHT 20MHz TX rate for non-AP STA

The 4-octet EHT MCS/NSS subfield is only used for 20 MHz-only
non-AP STA. Correct the interpretation of this subfield to
prevent improper rate limitations.

Fixes: f1dfcee2 ("wifi: rtw89: Correct EHT TX rate on 20MHz connection")
Signed-off-by: Kuan-Chung Chen <damon.chen@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250605114207.12381-6-pkshih@realtek.com