[radio] send max power table to radio (#5867)

This change sends the max power table to the radio so that the radio
chip can manage the transmit power by self.

tests/scripts/expect/posix-max-power-table.exp

@@ -37,6 +37,11 @@ expect "Done"
 send "channel preferred\n"
 expect "0x3fff800"
 expect "Done"
+send "txpower 20\n"
+expect "Done"
+send "txpower\n"
+expect "11 dBm"
+expect "Done"
 send "\x04"
 expect eof
 # allows all channels by default