dev/mana: replace power2 function

Replace is_power_of_2(length) with power2(length).  When length != 0, as in
this case, they produce the same result.  This will allow an implementation
of is_power_of_two to be dropped.

Reviewed by:	alc, markj
Differential Revision:	https://reviews.freebsd.org/D45536

(cherry picked from commit a94ed493b50752cee09245fc312c63b00331f217)

sys/dev/mana/gdma_main.c

@@ -939,7 +939,7 @@ mana_gd_create_dma_region(struct gdma_dev *gd,
 	int err;
 	int i;
 
-	if (length < PAGE_SIZE || !is_power_of_2(length)) {
+	if (length < PAGE_SIZE || !powerof2(length)) {
 		mana_err(NULL, "gmi size incorrect: %u\n", length);
 		return EINVAL;
 	}