rtw89: add const in the cast of le32_get_bits()

Add 'const' to be clear that this is a read-only access, and this patch
doesn't change logic at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Tested-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20211119054512.10620-3-pkshih@realtek.com