virtio_net: remove overzealous printk

The 'disable_cb' is really just a hint and as such, it's possible for more
work to get queued up while callbacks are disabled.  Under stress with an
SMP guest, this printk triggers very frequently.  There is no race here, this
is how things are designed to work so let's just remove the printk.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>