net: stmmac: FR60005 loopback disable

Add support to provide only 1 argument as part
of disable . Speed argument is not needed to disable
loopback.

Change-Id: I442bb61dccc17bed30014d0c3eca2e14cdf9a8fd
Signed-off-by: Suraj Jaiswal <jsuraj@codeaurora.org>