Bluetooth: btrtl: fix incorrect skb allocation failure check

Currently the check for a failed bt_skb_alloc allocation is incorrectly
checking using IS_ERR and this can lead to a null pointer dereference. Fix
this by checking for a null pointer return using the !skb idiom.

Addresses-Coverity: ("Dereference null return")
Fixes: 1996d9ca ("Bluetooth: btrtl: Ask 8821C to drop old firmware")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>