Write set and get eswitch mode functions used by devlink
ops. Use new pf struct member eswitch_mode to track current
eswitch mode in driver.

Changing eswitch mode is only allowed when there are no
VFs created.

Create new file for eswitch related code.

Add config flag ICE_SWITCHDEV to allow user to choose if
switchdev support should be enabled or disabled.

Use case examples:
- show current eswitch mode ('legacy' is the default one)
[root@localhost]# devlink dev eswitch show pci/0000:03:00.1
pci/0000:03:00.1: mode legacy

- move to 'switchdev' mode
[root@localhost]# devlink dev eswitch set pci/0000:03:00.1 mode
switchdev
[root@localhost]# devlink dev eswitch show pci/0000:03:00.1
pci/0000:03:00.1: mode switchdev

- create 2 VFs
[root@localhost]# echo 2 > /sys/class/net/ens4f1/device/sriov_numvfs

- unsuccessful attempt to change eswitch mode while VFs are created
[root@localhost]# devlink dev eswitch set pci/0000:03:00.1 mode legacy
devlink answers: Operation not supported

- destroy VFs
[root@localhost]# echo 0 > /sys/class/net/ens4f1/device/sriov_numvfs

- restore 'legacy' mode
[root@localhost]# devlink dev eswitch set pci/0000:03:00.1 mode legacy
[root@localhost]# devlink dev eswitch show pci/0000:03:00.1
pci/0000:03:00.1: mode legacy
Co-developed-by: Grzegorz Nitka <grzegorz.nitka@intel.com>
Signed-off-by: Grzegorz Nitka <grzegorz.nitka@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>