Bluetooth: Prioritize event packets over data packets

When there are packets available on both the event and data channels,
event packets must be read first. UART-based HCI controllers do this
prioritization before sending data over the serial connection, but
with SMD the HCI driver must serialize packets from the two channels.

CRs-fixed: 324277
Change-Id: Ic2b1ef21b2463b07eb20141f38f7cbfef1a9cd9c
Signed-off-by: Bhasker Neti <bneti@codeaurora.org>