auxdisplay: ht16k33: Make ht16k33_fb_fix and ht16k33_fb_var constant

The static structures ht16k33_fb_fix and ht16k33_fb_var, of types
fb_fix_screeninfo and fb_var_screeninfo respectively, are not used
except to be copied into other variables. Hence make both of them
constant to prevent unintended modification.
Issue found with
Coccinelle.

Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>