ixgbe: Advance version to 4.2.1

ixgbe: Advance version to 4.2.1

Orabug: 21918732

With the addition of X550em_x SFP+ support, the driver is now
functionally equivalent to what will be the 4.2.1 driver when
released, so change the version to match.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 21dd560162d4bbf98cc81b303bfa19740ae5b964)
Signed-off-by: Brian Maly <brian.maly@oracle.com>