vmxnet3: add support for get_coalesce, set_coalesce ethtool operations

The emulation supports a variety of coalescing modes viz. disabled
(no coalescing), adaptive, static (number of packets to batch before
raising an interrupt), rate based (number of interrupts per second).

This patch implements get_coalesce and set_coalesce methods to allow
querying and configuring different coalescing modes.

Signed-off-by: Keyong Sun <sunk@vmware.com>
Signed-off-by: Manoj Tammali <tammalim@vmware.com>
Signed-off-by: Shrikrishna Khare <skhare@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>