xtensa: fix ibreakenable register update

Only set the register when there is at least one ibreak register,
otherwise the build fails:
	arch/xtensa/kernel/head.S:105: Error: invalid register 'ibreakenable'
	for 'wsr' instruction
	arch/xtensa/platforms/iss/setup.c:67: Error: invalid register
	'ibreakenable' for 'wsr' instruction

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>