wifi: rtw89: sar: correct TX power boundary for MAC domain

TX power in MAC domain is signed 7 bits. (unit: based on txpwr_factor_mac)
The valid range should be [-64, 63].

While the original wrong bounds might not really be encountered, still make
them correct.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/20240412115729.8316-2-pkshih@realtek.com