xtensa: fix build fail on undefined ack_bad_irq

Commit e520c410

    "xtensa: convert to asm-generic/hardirq.h"

converted over to using the asm-generic parts, but it also
added the sentinel

	#define ack_bad_irq ack_bad_irq

which tells asm-generic to _not_ use the common ack_bad_irq.

Since e520c410

 deleted the duplicated code from the arch specific
file, we _do_ want the asm-generic one in scope.  So delete
the trigger define above which hides it.  In doing so we'll
realize that we've got to delete the almost-duplicate prototype
as well to avoid "static declaration ... follows non-static".

Cc: Christoph Hellwig <hch@lst.de>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>