netmap: add cast to fix powerpc64 LINT kernel

[emaste]

Summary

Attempt to fix powerpc64 LINT kernel broken by rS308000. Netmap's use of a uint64_t wchan seems odd, but in the interest of minimizing this change just cast through uintptr_t to silence the compiler warning.

(universe has been broken for a month due to this)

[jhb]

I can't find where wchan is actually assigned a value. Oh, it's gross:

dev/netmap/netmap_freebsd.c:            nmk->worker_ctx.cfg = *((struct ptnetmap_cfgentry_bhyve *)opaque);

(I guess userland gets to set the value? That's kind of interesting). I think this is fine because you cast the value on both sleep and wakeup, so if we lose the upper 32 bits we consistently lose them.