res/res_rtp_asterisk.c: Fix incorrect assignment of frame->subclass.frame_ending

In ast_rtp_read, the value of the variable 'mark' which we try to assign to a
frame->subclass.frame_ending may be 0, 1 or (1<<23), but we should translate
it to 0 or 1.

ASTERISK-25451 #close
Change-Id: I53bdf5c026041730184a6a809009c028549ce626