e1000: Report 82571 packet buffer ECC errorsHEADmain

The 82571 PBA_ECC register contains a 12-bit count of packet buffer ECC
detections.  The shared code enables single-bit correction, but neither
FreeBSD nor the DPDK base driver consumes the counter.

Sample it with the ordinary statistics timer, accumulate the value under
dev.em.N.memory_errors.detected_packet_buffer, and clear the hardware
counter while preserving correction and reserved register state.  Do not
enable its shared interrupt: the register does not distinguish corrected
from uncorrectable events and does not provide a safe fatal recovery
policy.

Validated on a dual port 82571EB.  Both functions reported zero after a
clean boot, and a controlled link down/up cycle left the counter at zero
while the management link recovered at 1 Gb/s without issue.

MFC after:	2 weeks
Sponsored by:	BBOX.io

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