IuUP_Emulation: Fix frame number wrap-around

As seen when running a test:
IuUP_Emulation.ttcn:150 Dynamic test case error: While RAW-encoding type '@IuUP_Types.IuUP_PDU': There are insufficient bits to encode '@IuUP_Types.IuUP_PDU_Type_0.frame_nr'

Change-Id: Ic9e69115c49545a7b9797d8f85568d5e91dfb329
1 file changed