[NET]: Fix busy waiting in dev_close().

If the current task has signal_pending(), the loop we have
to wait for the __LINK_STATE_RX_SCHED bit to clear becomes
a pure busy-loop.

Fixed by using msleep() instead of the hand-crafted version.

Noticed by Andrew Morton.

Signed-off-by: David S. Miller <davem@davemloft.net>