Add a helper function to fetch ffa_dev's driver_data using
dev_get_drvdata. At the same time move existing ffa_dev_set_drvdata
to use dev_set_drvdata.

Link: https://lore.kernel.org/r/20220429113946.2087145-3-sudeep.holla@arm.comSuggested-by: Arunachalam Ganapathy <arunachalam.ganapathy@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>