rtlwifi: rtl818x: remove redundant check for cck_power > 15

cck_poweri cannot be greated than 15 as is derived from the bottom 4 bits
from riv->channels[channel - 1].hw_value & 0xf.  Hence the check for it
being greater than 15 is redundant and can be removed.

Detected by CoverityScan, CID#744303 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>