Staging: hv: mousevsc: Get rid of the usage of the ext field in struct hv_device

Get rid of the usage of the ext field in struct hv_device for the mouse driver.
We do this by using the newly introduced functions to set and and get driver
specific data.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>