staging: panel: fix sparse warnings in lcd_write

This patch fixes two sparse warnings related to lcd_write :
warning: incorrect type in argument 1 (different address spaces)
warning: incorrect type in initializer (incompatible argument 2
(different address spaces))

lcd_write can be used from kernel space (in panel_lcd_print) or from user
space. So we introduce the lcd_write_char function that will write a char to
the device. We modify lcd_write and panel_lcd_print to use it. Finally we add
__user annotation missing in lcd_write.

Signed-off-by: Bastien Armand <armand.bastien@laposte.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>