net/igc: fix Rx packet size

net/igc: fix Rx packet size

When DEV_RX_OFFLOAD_KEEP_CRC is enabled, the PMD will minus 4 bytes
of CRC from the size of a packet, but the NIC will strip the CRC
because the CRC strip bit in DVMOLR register is not cleared. This
will cause the size of a packet to be 4 bytes less.

This patch updates the CRC strip bit according to whether
DEV_RX_OFFLOAD_KEEP_CRC is enabled.

Fixes: a5aeb2b9e225 ("net/igc: support Rx and Tx")
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Acked-by: Haiyue Wang <haiyue.wang@intel.com>