wifi: rtw89: 8851b: rfk: remove unnecessary assignment of return value of _dpk_dgain_read()

The return value of _dpk_dgain_read() is not used afterward, so remove
it safely.

Addresses-Coverity-ID: 1504753 ("Unused value")

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20241209042020.21290-2-pkshih@realtek.com