serial: Make ucc_uart work in HW UART mode

In HW UART mode the TxBD[READY] is not cleared by H/W (RISC engine) when
the user send characters to Tx buffer of QE UART.  So, these characters
stay on the QE forever, never go to UART line.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>