ixgbe: refactor common start_hw code for 82599 and x540

Factored out the common start_hw code into a new function
ixgbe_start_hw_gen2() so that it can be used by x540 and 82599.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Acked-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Evan Swanson <evan.swanson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>