spi: cadence-xspi: use for_each_available_child_of_node_scoped()

spi: cadence-xspi: use for_each_available_child_of_node_scoped()

Refactor code for "is the node's child available?" check by using the
corresponding macro instead, which reads more clearly.

While at it, use scope-based cleanup instead of manual of_node_put()
calls when getting platform data through cdns_xspi_of_get_plat_data().

This removes the unnecessary "node_child" declaration out of the loop's
scope and auto cleans up "node_child" when it goes out of scope, even
when we return early due to error.

Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Link: https://msgid.link/r/20240410130205.179069-1-five231003@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>