serial: arc_uart: Use uart_circ_empty() for open-coded comparison

Replace open-coded test for empty tx ring buffer with equivalent
helper function, uart_circ_empty(). No functional change.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>