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

Fix warnings:
flash.c: In function 'write_word_1':
flash.c:696: warning: dereferencing type-punned pointer will break strict-aliasing rules
flash.c: In function 'write_word_2':
flash.c:1044: 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>