ixgbe: Add feature offset value to ring features

The mask value for ring features was overloaded for FCoE which can lead to
some confusion.  In order to avoid any confusion I am splitting the mask
value and adding an offset value.  This can be used for the start of the
FCoE rings, and in the future I hope to use it to store the start of the
registers for SR-IOV.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>