cc2520: fix CC2591 handling

This patch changes tha way of handling of cc2591-cc2520 combination
by moving amplified variable from platform data to private data.
This will be useful in other sections like tx power support.

Signed-off-by: Varka Bhadram <varkab@cdac.in>
Cc: Brad Campbell <bradjc5@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>