rtw88: Fix an error message

The WARN_ON() macro takes a condition, not a warning message.  I've
changed this to use WARN() instead.

Fixes: 4136214f ("rtw88: add BT co-existence support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>