video: enum comparison to constant fix

Set the maximum for reserved values in hdmi_spd_sdi and
hdmi_audio_coding_type_ext enums and use these max enum values
in comparisons instead of constants. This fixes the warning
generated when compiling with LLVM using the -Wenum-compare
option.

Change-Id: I2cc442f33d98a86030ff5fa0f969bbf3682394b3
Signed-off-by: Steve Cohen <cohens@codeaurora.org>