SMU is the safest way to set it.

# smu_cli set tx-max-bg 6

# smu_cli get tx-max-bg

    Reg Domain:  WW

    BG Antenna Adj: 3 dB

    A  Antenna Adj: 0 dB

Remember the  value is in ½ db increments so you need to use 6 to get 3 dBm. 


Also fw_setenv is able to set it as well.
rfPowerAtten2=6
 

# fw_printenv

...

usbnet_devaddr=de:ad:be:ef:00:01

md5_at91bs=75a5a900d8df5657ebc89796756eed70

md5_u-boot=cc8187cf4471832e51580a284df37f86

md5_kernel-a=300d5b9d93abaec368fb700933a95bdb

md5_rootfs-a=62e98ead848d552ab6199b6ed543dd4d

bootargs=console=ttyS0,115200 loglevel=4 rw noinitrd mem=64M rootfstype=ubifs root=ubi0:rootfs ubi.mtd=6

bootcmd=nand read 0x22000000 0x000e0000 0x00500000; bootm

rfPowerAtten2=6