diag: hsic: Fix improper opening/closing of diag_bridge

diag_bridge_open/close should not be called multiple times, as it
would incorrectly increase/decrease the internal reference counter.
This could respectively lead to either a stale device structure when
HSIC is disconnected, or a NULL pointer when HSIC is reconnected.

Fix this by adding a check in the bridge open routine to disallow
further calls if the bridge is already open, and similarly in close
to no-op if the bridge isn't open. Also add a mutex to the diagfwd
driver to serialize access to the various state variables such as
hsic_device_enabled and hsic_device_opened and to prevent near-
simultaneous calls to diag_bridge_open. In particular, the platform
device probe (or remove) and the callback for cable connect
(disconnect) could potentially execute concurrently; only one should
"win" calling diag_bridge_open (or close) successfully.

CRs-fixed: 395637
Change-Id: I5674285065cd63f89bf657cad770f04d53efb247
Signed-off-by: Jack Pham <jackp@codeaurora.org>