serial: sandbox: support Unicode

Due to a conversion error the sandbox does not accept byte values 0x80-0xff
from the keyboard. The UEFI extended text input unit test requires Unicode
support.

Use unsigned char for the serial buffer.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>

drivers/serial/sandbox.c

@@ -33,7 +33,7 @@ DECLARE_GLOBAL_DATA_PTR;
  *   serial_buf_write		 == serial_buf_read -> empty buffer
  *   (serial_buf_write + 1) % 16 == serial_buf_read -> full buffer
  */
-static char serial_buf[16];
+static unsigned char serial_buf[16];
 static unsigned int serial_buf_write;
 static unsigned int serial_buf_read;