aaudio: fix FIFO wrapround frame counts

The number of frames in the second part was incorrect
when it wrapped past the end of the FIFO.

Bug: 78139448
Test: adb shell aaudio_loopback -c2 -C2 -tm -m2 -Pl -pn
Change-Id: Ib6a22f30f0ee5321444c6d6137d9488943c34f3d