mxfdec: update current edit unit when seeking in mxf files without index tables

Fixes audio packet pts values in some files generated by AVID TRMG 3.01.

Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Tomas Härdin <tomas.hardin@codemill.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>