include/asm-alpha/io_trivial.h build fixes

This patch cherry picks the following from
commit 9548b209:

fix build failure with gcc-4.2.x: fix up casts in cia_io* routines to avoid
warnings ('discards qualifiers from pointer target type'), which are
failures, thanks to -Werror;

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Adrian Bunk <bunk@kernel.org>