Staging: panel: Use u8 type

Declare om, im, omask and imask as u8 to remove any confusion if
that describes the 8 bits of the data bus on the parallel port.
Also change return type of lcd_write_data() to u8.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>