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: 0830d72b ("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>