of_serial: 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: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Matthias Fuchs <mfuchs@ma-fu.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>