spl: nand: read only in the size of image_header on the first access

For the same reason as in commit 50c8d66d, all the remaining
CONFIG_SYS_NAND_PAGE_SIZE in common/spl/spl_nand.c can be replaced
with sizeof(*header).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>