cc2520: update initial transmit power value

CC2520 has the default 0dBm transmit power level  on reset.
This patch update initial value of transmit power with 0dBm value.

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