tty: xuartps: Fix "may be used uninitialized" build warning

Initialize varibles for which a 'may be used uninitalized' warning is
issued.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>