emulator: A minor cleanup in goldfish_dma

This change makes fields of fixed length to make safe to pass
goldfish_dma_context between 32 and 64 bit environments.

Test: ~/repo-emu-master/external/qemu/objs/emulator \
Test:     -verbose -show-kernel \
Test:     -kernel /tmp/qemu-kernel-x86_64-ranchu/kernel-qemu \
Test:     -feature GLDMA -wipe-data -no-snapshot
Bug: 110152998
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: I8e2aaf2d6896f189251099dabdc49d86053b64c6