drivers: ieee802154: cc2520: Rename RX stack Kconfig option

Fibers do not exist anymore in unified kernel.
Also, let's just use that option directly so removing the internal
define which is then useless.

Change-Id: I1179d84401934b339d3a9ccd12f6145a120bcb2f
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>