netmap: add cast to fix powerpc64 LINT kernel

Attempt to fix powerpc64 LINT kernel broken by r308000. 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.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8669