cc2520: Do not store platform_data in spi_device

Storing the `platform_data` struct inside of the SPI struct when using
the device tree allows for a later function to edit the content of that
struct. This patch refactors the `cc2520_get_platformat_data` function
to accept a pointer to a `cc2520_platform_data` struct and populates
the fields inside of it.

This change mirrors commit aaa1c4d2
("at86rf230: copy pdata to driver allocated space").

Signed-off-by: Brad Campbell <bradjc5@gmail.com>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>