ifo_types: avoid forcing a higher length in bitfield structures

LLVM ends up using the size of an unsigned int if it's used in the bitfield. In other places the bitfields are using unsigned char which always uses the proper size in the packed structures.

• multichannel_ext_t would end up having a size of 27 instead of 24
• cell_playback_t would end up having a size of 26 instead of 24
• pgci_srp_t would end up having a size of 11 instead of 8
• playback_type_t would end up having a size of 4 instead of 1

v2:

• do the same fix in nav_types.h

v3:

• btni_t is not read as a packed structure and cannot use unsigned char with 10 bits fields http://dvd.sourceforge.net/dvdinfo/pci_pkt.html#btn_it
• This is the reason dvdnav doesn't work with mismatching MS bitfield packing