wifi: rtw89: phy: fix out-of-bounds access in rtw89_phy_read_txpwr_limit()

wifi: rtw89: phy: fix out-of-bounds access in rtw89_phy_read_txpwr_limit()

Coverity reported a potential out-of-bounds access when 'bw' exceeds the
valid range for the specified band. Add a helper `rtw89_bw_is_valid()`
to check bandwidth validity for each band before accessing limit tables.

Addresses-Coverity-ID: 1598844 ("Out-of-bounds access")
Addresses-Coverity-ID: 1598896 ("Out-of-bounds access")

Signed-off-by: Kuan-Chung Chen <damon.chen@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20251111022452.28093-6-pkshih@realtek.com