net/bnxt: support set MTU

This patch adds support to modify MTU using the set_mtu dev_op.
To support frames > 2k, the PMD creates an aggregator ring.
When a frame greater than 2k is received, it is fragmented
and the resulting fragments are DMA'ed to the aggregator ring.
Now the driver can support jumbo frames upto 9500 bytes.

Signed-off-by: Steeven Li <steeven.li@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>