libpayload: avoid excessive casts in printf.c

libpayload: avoid excessive casts in printf.c

struct printf_spec is a purely internal structure. Avoid excessive casts
when using the write function pointer just to make the compiler happy by
using the right types in the first place.

Change-Id: Ia4f3c79a5283cb76c8aa5f9d1eee758676303382
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: http://review.coreboot.org/850
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>