blackfin: Fix tty compile error in PIO mode

Fixes this compile issue:

  drivers/serial/bfin_5xx.c: In function bfin_serial_rx_chars:
  drivers/serial/bfin_5xx.c:178: error: struct uart_info has no member named tty

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>