Bluetooth: Fix minor coding style issue in smp.c

The convention for checking for NULL pointers is !ptr and not
ptr == NULL. This patch fixes such an occurrence in smp.c.

Signed-off-by: Johan Hedberg <[e-mail]>
Signed-off-by: Marcel Holtmann <[e-mail]>

net/bluetooth/smp.c

···
 	uint8_t tmp[16], data[16];
 	int err;
 
-	if (tfm == NULL) {
+	if (!tfm) {
 		BT_ERR("tfm %p", tfm);
 		return -EINVAL;
 	}