pch_uart: change type to u8

Target uart register access size is 8bit.
However, 32bit is used at 2 points.

This patch modifies type "unsigned int" to "unsigned char".

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>