avcodec/hevc_sei: fix amount of bits skipped when reading picture timing SEI message

The code was skipping the entire reported SEI message size regardless of
the amount of bits read.
While in theory safe for NALU where the picture timing SEI message is alone
or at the end as we're using the checked bitstream reader, it isn't in any
other situation, where every SEI message in the NALU after the picture
timing one would potentially fail to parse.

Change the function name to one more in line with the rest of file, and
remove the bogus "Skipped SEI" debug message while at it.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>
