HID: picoLCD: constify fb ops

Now that the fbops member of struct fb_info is const, we can start
making the ops const as well.

v2: fix	typo (Christophe de Dinechin)

Cc: Bruno Prémont <bonbons@linux-vserver.org>
Cc: linux-input@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Bruno Prémont <bonbons@linux-vserver.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f415010cd52cce7d8a1250d4eca582ec64e67956.1575390741.git.jani.nikula@intel.com