powerpc/5200: Remove pr_debug() from hot paths in irq driver

pr_debug() calls in the 'hot' *_mask(), *_unmask(), *_ack() and
get_irq() makes adding #define DEBUG pretty much useless.  Remove
these calls because they completely swamp the output.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>