net: stmmac: selftests: Add Split Header test

Add a test to validate that Split Header feature is working correctly.
It works by using the rececently introduced counter that increments each
time a packet with split header is received.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>