wifi: rtw89: coex: check NULL return of kmalloc in btc_fw_set_monreg()

kmalloc may fail, return value might be NULL and will cause
NULL pointer dereference. Add check NULL return of kmalloc in
btc_fw_set_monreg().

Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Fixes: b952cb0a ("wifi: rtw89: coex: Add register monitor report v7 format")
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/516a91f3997534f708af43c7592cbafdd53dd599.1730253508.git.xiaopei01@kylinos.cn