ethtool: strset: fix header attribute index in ethnl_req_get_phydev()

ethtool: strset: fix header attribute index in ethnl_req_get_phydev()

strset_prepare_data() passes ETHTOOL_A_HEADER_FLAGS (3) as the header
attribute to ethnl_req_get_phydev(). This is incorrect, in the main
attr space 3 is ETHTOOL_A_STRSET_COUNTS_ONLY, not the request
header attr. The correct constant is ETHTOOL_A_STRSET_HEADER (1).

ethnl_req_get_phydev() only uses this value for the extack,
so this is not a "functionally visible"(?) bug.

Fixes: e96c93aa4be9 ("net: ethtool: strset: Allow querying phy stats by index")
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260526153533.2779187-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>