Staging: remove driver_data direct access of struct device

In the near future, the driver core is going to not allow direct access
to the driver_data pointer in struct device.  Instead, the functions
dev_get_drvdata() and dev_set_drvdata() should be used.  These functions
have been around since the beginning, so are backwards compatible with
all older kernel versions.

Cc: Leon Woestenberg <leon@sidebranch.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>