selftests: net: Add port split test

Test port split configuration using previously added number of port lanes
attribute.

Check that all the splittable ports are successfully split to their maximum
number of lanes and below, and that those which are not splittable fail to
be split.

Test output example:

TEST: swp4 is unsplittable                                         [ OK ]
TEST: split port swp53 into 4                                      [ OK ]
TEST: Unsplit port pci/0000:03:00.0/25                             [ OK ]
TEST: split port swp53 into 2                                      [ OK ]
TEST: Unsplit port pci/0000:03:00.0/25                             [ OK ]

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>