net: gianfar: Use device_get_named_child_node_count()

We can avoid open-coding the loop construct which counts firmware child
nodes with a specific name by using the newly added
device_get_named_child_node_count().

The gianfar driver has such open-coded loop. Replace it with the
device_get_child_node_count_named().

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/3a33988fc042588cb00a0bfc5ad64e749cb0eb1f.1750248902.git.mazziesaccount@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>