arm: am33xx: security: Fix size calculation on header

Fix the size calculation in the verify boot. The header size
should be subtracted from the image size, not be assigned to
the image size.

Fixes: 0830d72bb9f8 ("arm: am33xx: security: adds auth support for encrypted images")
Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>

arch/arm/mach-omap2/sec-common.c

@@ -130,7 +130,7 @@ int secure_boot_verify_image(void **image, size_t *size)
 	*size = sig_addr - cert_addr;	/* Subtract out the signature size */
 	/* Subtract header if present */
 	if (strncmp((char *)sig_addr, "CERT_ISW_", 9) == 0)
-		*size = ((u32 *)*image)[HEADER_SIZE_OFFSET];
+		*size -= ((u32 *)*image)[HEADER_SIZE_OFFSET];
 	cert_size = *size;
 
 	/* Check if image load address is 32-bit aligned */