ixgbe: make device_caps() generic

x540 has the same device capability word in the EEPROM as 82599.
This patch renames ixgbe_get_device_caps_82599 to
ixgbe_get_device_caps_generic, moves it to ixgbe_common.h and
sets up the function pointer for x540.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Evan Swanson <evan.swanson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>