tty/serial/8250: fix RS485 half-duplex RX

When in half-duplex mode RX will be disabled before TX, but not
enabled after deactivating transmitter. This patch enables
UART_IER_RLSI and UART_IER_RDI interrupts after TX is over.

Cc: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Fixes: e490c914 ("tty: Add software emulated RS485 support for 8250")
Acked-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>