igc: Add sysctl for DMA Coalesce

This feature can increase efficiency at the expense of latency

It does not work well with the default interrupt delay, but expose
the otherwise unconnected code in the driver in case people want to
experiment.

See
https://www.intel.com/content/dam/support/us/en/documents/network/adapter/pro100/sb/466827_intel_r__dma_coalescing_white_paper_v003.pdf

MFC after: 1 week
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