cc2520: use devm_kzalloc(.., sizeof(*pointer), ..) pattern

Signed-off-by: Varka Bhadram <varkab@cdac.in>
cc2520: remove 'ret' goto label

If allocation of memory fails instead of going to ret goto label
and returning from there, we can directly return -ENOMEM on failure.

Signed-off-by: Varka Bhadram <varkab@cdac.in>
cc2520: fix zero perm_extended_addr address