drm/imx: tve remove extraneous type qualifier

clang warns about functions returning a 'const int' result:

drivers/gpu/drm/imx/imx-tve.c:487:8: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]

Remove the extraneous 'const' qualifier here. I would guess that the
function was intended to be marked __attribute__((const)) instead,
but that would also be wrong since it call other functions without
that attribute.

Fixes: fcbc51e5 ("staging: drm/imx: Add support for Television Encoder (TVEv2)")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>