ixgbe: change mac_type if statements to switch statements

This change replaces a number of if/elseif/else statements with switch
statements to support the addition of future devices to the ixgbe driver.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>