e1000: correctly set isc_pause_frames only when XOFF increases

From Jake:
The e1000 driver sets the iflib shared context isc_pause_frames value to
the number of received xoff frames. This is done so that the iflib
watchdog timer won't trigger a Tx Hang due to pause frames.

Unfortunately, the function simply sets it to the value of the xoffrxc
counter. Once the device has received a single XOFF packet, the driver
always reports that we received pause frames. This will prevent the Tx
hang detection entirely from that point on.

Fix this by assigning isc_pause_frames to a non-zero value if we
received any XOFF packets in the last timer interval.

We could attempt to calculate the total number of received packets by
doing a subtraction, but the iflib stack only seems to check if
isc_pause_frames is non-zero.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed by: gallatin@
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21868