The tx_mask check doesn't reflect what the driver and the chip support.

The check currently checks for exactly two slots being enabled. The
tlv320adcx140 supports anything between one and eight channels, so relax
the check accordingly.

The tlv320adcx140 supports arbitrary tx_mask settings, but the driver
currently only supports adjacent slots beginning with the first slot,
so extend the check to check that the first slot is being used and that
there are no holes in the tx_mask.

Leave a comment to make it's the driver that limits the tx_mask
settings, not the chip itself.

While at it remove the set-but-unused struct adcx140p_priv::tdm_delay
field.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20220624105716.2579539-1-s.hauer@pengutronix.deSigned-off-by: Mark Brown <broonie@kernel.org>