tee: optee: Use ffa_dev_get_drvdata to fetch driver_data

Due to lack of an helper like ffa_dev_get_drvdata, this driver was
fetching driver_data directly accessing the structure member. Now that
we have added an helper, just use the same instead.

Link: https://lore.kernel.org/r/20220429113946.2087145-4-sudeep.holla@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>