ixgbe: Move bridge mode from flag to variable

We are currently storing our BRIDGE_MODE as a bit in our adapter flags.
This patch will store the actual mode instead which minimizes obfuscation
and makes following patches for X550 simpler.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>