alpha: fixup BUG macro

Do usual do {} while (0) dance, otherwise

fs/gfs2/util.c:99: error: expected expression before 'else'
drivers/scsi/lpfc/lpfc_sli.c:363: error: expected expression before 'else'

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: 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>