cc2520: Reworking radio on and transmission logic

FIFOP setting is a wrong assumption to know if radio is on.
Transmission runs on clock basis instead of pre-set loop counter. Thus
removing useless config option relevantly as well.
Opmitizing a bit the code as well, and making cc2520_on() public for
testing purpose.

Change-Id: I4495d1d6c19d10dcbc18f7e2fd5041720ec1f438
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>