staging: panel: Make two more module params read-only

Make keypad_type and lcd_type module params read-only.
This step also starts making it more clear what is
the precedence of device params coming from different
sources (device profile, runtime module param values etc).

Signed-off-by: Mariusz Gorski <marius.gorski@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>