net: dp83640: fix improper double spin locking.

A pair of nested spin locks was introduced in commit 63502b8d
"dp83640: Fix receive timestamp race condition".

Unfortunately the 'flags' parameter was reused for the inner lock,
clobbering the originally saved IRQ state.  This patch fixes the issue
by changing the inner lock to plain spin_lock without irqsave.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>