drivers: net: xgene: Check for IS_ERR rather than NULL for clock.

This patches fixes the code to check for IS_ERR rather
than NULL for clock interface.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>