intr/powerpc: powerpc_dispatch_intr() return stray count

[ehem_freebsd_m5p.com]

Summary

Since the caller could be interested in the stray count, return the
stray count if a stray has occurred. In case of normal interrupt
handling, return 0.

[jhibbits]

Note, stray_count is the *total* strays, not just the strays from the current call.

[ehem_freebsd_m5p.com]

That is a bit of an oddity compared with other architectures, but it should still provide equivalent results.