avformat/flvdec: read the correct bits into the tag type

from the flv spec, the flvtag define the tagtype as one byte,
the spec desc is:
Reserved  UB[2] Reserved for FMS, should be 0
Filter    UB[1] Indicates if packets are filtered.
                0 = No pre-processing required.
                1 = Pre-processing (such as decryption) of the packet is
                    required before it can be rendered.
                    Shall be 0 in unencrypted files, and 1 for encrypted
tags.
                    See Annex F. FLV Encryption for the use of filters.
TagType   UB[5] Type of contents in this tag. The following types are
                defined:
                        8 = audio
                        9 = video
                        18 = script data

Signed-off-by: Steven Liu <qi.liu@chinacache.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>