ixgbe: change MAX_RXD/MAX_TXD based on adapter type

Set the length limit for the receive descriptor buffer and transmit
descriptor buffer based on the controller type. The values used are called
out in the controller datasheets as a 'Note:' in the RDLEN and TDLEN
register descriptions.

This allows the user to use ethtool to allocate larger descriptor buffers
in the case where data is received or transmitted too quickly for the
driver to keep up.

Signed-off-by: Daniel Willenson <daniel@veobot.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>