cc2520: fix in default tx power setting

Initially we dont have the tx power settings from the user-space.
Now we have the support for seeting the tx power level.

So lets use the default tx power setting for the radio.

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