Staging: hv: blkvsc: fix up driver_data usage

driver_data is gone now from struct device, so use the proper functions
to access it instead.

Thanks to Bill Pemberton for pointing out this build error.

Cc: Bill Pemberton <wfp5p@viridian.itc.Virginia.EDU>
Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>