nic_uio: fix thread structure compatibility for future FreeBSD

nic_uio: fix thread structure compatibility for future FreeBSD

Replace d_thread_t with struct thread in nic_uio.

Ref: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196691
Quote:
"The d_thread_t typedef is a compat shim to support FreeBSD 4.x.
I'm planning to remove this shim from 11 and dpdk is very unlikely
to ever be ported to 4.x.
If it does it will need far more changes than just d_thread_t"

Reported-by: John Baldwin <jhb@freebsd.org>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>