openrisc/boot: Remove unnecessary initialisation in memcpy().

'd' and 's' are initialised later with 'dest_w' and 'src_w', so we need not
initialise them before that.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Stafford Horne <shorne@gmail.com>