ethtool: tsconfig: reject zero-valued tx_type and rx_filter bitsets

The ffs()/fls() guard in ethnl_set_tsconfig() was meant to enforce
that the user selects exactly one tx_type (and one rx_filter)
at a time (off / none are explicit types with non-zero values).
However, both ffs(0) and fls(0) return 0, so the guard passes
a zero-valued bitset through.

The subsequent ffs(req_tx_type) - 1 would produce -1, if user selected
no bit. net_hwtstamp_validate() catches the invalid -1 downstream,
but returns a generic error (-ERANGE) without telling the user
what went wrong. Return -EINVAL + extack instead.

Replace the ffs()/fls() comparison with a hweight32() == 1 check.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Joe Damato <joe@dama.to>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20260812162230.1837788-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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