igb: change pba size determination from if to switch statement

As additional hardware is added to the igb driver it is easier to support
the expansion via switch statements instead of using nested ifs.  For
this reason I am changing this to a switch statement.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>