msm: gsi: use LSB to ring doorbells

MSB is always fixed once the channel is setup. Ring
doorbell on for new descriptors by writing to LSB register only.

CRs-Fixed: 2202013
Change-Id: If4e72c5e6c4c9cc58dafae8a44e5f389be593333
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>