tty: serial: Initialized rxbs_status

tty: serial: Initialized rxbs_status

This patch removes compilation warning:
drivers/tty/serial/xilinx_uartps.c: In function ‘cdns_uart_isr’:
drivers/tty/serial/xilinx_uartps.c:313:21: warning: ‘rxbs_status’ may be
used uninitialized in this function [-Wmaybe-uninitialized]
    if ((rxbs_status & CDNS_UART_RXBS_FRAMING)
                     ^
drivers/tty/serial/xilinx_uartps.c:245:15: note: ‘rxbs_status’ was
declared here
  unsigned int rxbs_status;

Signed-off-by: Michal Simek <michal.simek@xilinx.com>