wifi: rtw88: phy: fix warning of possible buffer overflow

reported by smatch

phy.c:854 rtw_phy_linear_2_db() error: buffer overflow 'db_invert_table[i]'
8 <= 8 (assuming for loop doesn't break)

However, it seems to be a false alarm because we prevent it originally via
       if (linear >= db_invert_table[11][7])
               return 96; /* maximum 96 dB */

Still, we adjust the code to be more readable and avoid smatch warning.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220727065003.28340-5-pkshih@realtek.com