rt2800: comment and simplify AGC init for RT6352

We do not need separate lines for calculating register values.
Also add comment that value is different than in vendor driver.

Suggested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>