alpha: agp: make empty macros use do-while-0 style

Copy these macros from ia64/include/asm/agp.h to avoid the
"empty-body" in 'if' statment warning.

drivers/char/agp/generic.c: In function 'agp_generic_destroy_page':
../drivers/char/agp/generic.c:1265:42: warning: suggest braces around empty body in an 'if' statement [-Wempty-body]
 1265 |                 unmap_page_from_agp(page);

Link: https://lkml.kernel.org/r/20210809030822.20658-1-rdunlap@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>