Eliminate deadlock when writing config from radio link

Writing the 'Saved\r\n' string would fill the packet buffer and cause
a flush to occur, which would need to wait for the radio link to
receive and transmit a packet. The radio link always re-fetches the
radio channel number when lighting up the radio, so it need to look in
the config space. If the config mutex was held by the config writing
process while the radio was trying to get the channel number, then
we'd get a deadlock.

Signed-off-by: Keith Packard <keithp@keithp.com>