alpha: fix boot/main.c warning

In current 2.6.23-rc1+git, make bootimage gives the following warning while
compiling arch/alpha/boot/main.c.  The patch below fixes the warning by
casting callback argument explicitly to void*.  The original value comes from
START_ADDR macro and is clearly numeric so only cast it for the callback.

  CC      arch/alpha/boot/main.o
arch/alpha/boot/main.c: In function 'load':
arch/alpha/boot/main.c:135: warning: passing argument 3 of 'callback_read' makes pointer from integer without a cast

Signed-off-by: Meelis Roos <mroos@linux.ee>
Cc: Jay Estabrook <jay.estabrook@hp.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>