board/amcc/common/flash.c: Fix compile warning

Fix warning: ../common/flash.c:917: warning: dereferencing type-punned
pointer will break strict-aliasing rules

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>