net/failsafe: fix FreeBSD build

The type pthread_t is not portable because it is freely defined.
On Linux, it is an unsigned long int which can be printed with %l.
On FreeBSD, it is a pointer which can be printed with %p.

That's why there was this error:
drivers/net/failsafe/failsafe_private.h:377:53: error:
	format specifies type 'unsigned long' but the argument has
	type 'pthread_t' (aka 'struct pthread *')

Fixes: 655fcd68c7d2 ("net/failsafe: fix hotplug races")

Reported-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Tested-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Matan Azrad <matan@mellanox.com>