staging/panel: Mark local functions/structs static and add const if applicable (fix sparse warnings)

sparse complains about the following functions:
panel.c:188:1: warning: symbol 'logical_inputs' was not declared. Should it be static?
panel.c:569:6: warning: symbol 'old_keypad_profile' was not declared. Should it be static?
panel.c:580:6: warning: symbol 'new_keypad_profile' was not declared. Should it be static?
panel.c:593:6: warning: symbol 'nexcom_keypad_profile' was not declared. Should it be static?
panel.c:672:6: warning: symbol 'pin_to_bits' was not declared. Should it be static?
panel.c:1375:6: warning: symbol 'panel_lcd_print' was not declared. Should it be static?
panel.c:1382:6: warning: symbol 'lcd_init' was not declared. Should it be static?
panel.c:2181:5: warning: symbol 'panel_init' was not declared. Should it be static?

Add the static keyword to silence these warnings and make sparse happy.

If structs or function parameters are used readonly they are also marked
as const.

CC: David Howells <dhowells@redhat.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>