Bluetooth: uart: Remove dead code

There's no point in having a return value for uart_out() if it always
returns the same length that was passed to it. Just make it return
void and remove the error handling code from the caller (which would
never have been run).

Change-Id: I88dab7499a5f526ca65058640021f84e1b3486ca
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>