spl: spl_legacy: Fix NAND boot on OMAP3 BeagleBoard

OMAP3 BeagleBoard NAND boot hangs when spl_load_legacy_img() tries
to read the header into 'struct hdr' which is allocated on the
stack.

As the header has already been read once before by spl_nand.c,
we can avoid the extra header allocation and read here by
simply passing around the pointer to the header.

This fixes NAND boot on OMAP3 BeagleBoard.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-By: Michael Trimarchi <michael@amarulasolutions.com>