net: lan743x: reject unsupported external timestamp requests

The lan743x_ptp_io_event_cap_en() function checks that the given request
sets only one of PTP_RISING_EDGE or PTP_FALLING_EDGE, but not both.

However, this driver does not check whether other flags (such as
PTP_EXT_OFF) are set, nor whether any future unrecognized flags are set.

Fix this by adding the appropriate check to the lan743x_ptp_io_extts()
function.

Fixes: 60942c39 ("net: lan743x: Add support for PTP-IO Event Input External Timestamp (extts)")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250312-jk-net-fixes-supported-extts-flags-v2-3-ea930ba82459@intel.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>