net: stmmac: do not silently change auxiliary snapshot capture channel

Even though the hardware theoretically supports up to 4 simultaneous
auxiliary snapshot capture channels, the stmmac driver does support only
a single channel to be active at a time.

Previously in case of a PTP_CLK_REQ_EXTTS request, previously active
auxiliary snapshot capture channels were silently dropped and the new
channel was activated.

Instead of silently changing the state for all consumers, log an error
and return -EBUSY if a channel is already in use in order to signal to
userspace to disable the currently active channel before enabling another one.

Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>