rtw88: don't treat NULL pointer as an array

I'm not a standards expert, but this really looks to be undefined
behavior, when chip->dig_cck may be NULL. (And, we're trying to do a
NULL check a few lines down, because some chip variants will use NULL.)

Fixes: fc637a86 ("rtw88: 8723d: Set IG register for CCK rate")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821211716.1631556-1-briannorris@chromium.org