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: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>