spi: allow ancillary devices to share parent's chip selects

spi: allow ancillary devices to share parent's chip selects

When registering an ancillary SPI device, the current code flags a chip
select conflict with the parent device. This happens because the
ancillary device intentionally uses one of the parent's chip selects,
but __spi_add_device() checks against all existing devices including
the parent.

Allow this by passing the parent device pointer to __spi_add_device()
and skipping the conflict check when the existing device is the parent.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/bcb1eb34fc5e86fd5dbb4472ad1d3ea3cf3e9779.1770393792.git.antoniu.miclaus@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>