wifi: rtl8xxxu: Define masks for cck_agc_rpt bits

Define the constants CCK_AGC_RPT_LNA_IDX_MASK and
CCK_AGC_RPT_VGA_IDX_MASK instead of using the same literals
in four places.

And get the bits from cck_agc_rpt using u8_get_bits().

It's a cosmetic change only.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/cfe79922-efdf-2ed0-7404-263915d19d82@gmail.com