powerpc/powernv: Check image loaded or not before calling flash

Present code checks for update_flash_data in opal_flash_term_callback().
update_flash_data has been statically initialized to zero, and that
is the value of FLASH_IMG_READY. Also code update initialization happens
during subsys init.

So if reboot is issued before the subsys init stage then we endup displaying
"Flashing new firmware" message.. which may confuse end user.

This patch fixes above described issue by initializes update_flash status
to invalid state.

Reported-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>