omap/spl: change output of spl_parse_image_header

This only outputs "Assuming u-boot.bin..." if debug is active.

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
CC: Tom Rini <tom.rini@gmail.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Wolfgang Denk <wd@denx.de>