misc/mei: Add NULL check to component match callback functions

Currently, component_match callback functions used in mei refers to the
driver name, assuming that the component device being matched has a
driver bound. It can cause a NULL pointer dereference when a device
without a driver bound registers a component. This is due to the nature
of the component framework where all registered components are matched
in any component_match callback functions. So even if a component is
registered by a totally irrelevant device, that component is also
shared to these callbacks for i915 driver.

To prevent totally irrelevant device being matched for i915 and causing
a NULL pointer dereference for checking driver name, add a NULL check on
dev->driver to check if there is a driver bound before checking the
driver name.

In the future, the string compare on the driver name, "i915" may need to
be refactored too.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Won Chung <wonchung@google.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>