rtw88: fix potential read outside array boundary

The level of cckpd is from 0 to 4, and it is the index of
array pd_lvl[] and cs_lvl[]. However, the length of both arrays
are 4, which is smaller than the possible maximum input index.
Enumerate cck level to make sure the max level will not be wrong
if new level is added in future.

Fixes: 479c4ee9

 ("rtw88: add dynamic cck pd mechanism")
Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>