USB: sisusbvga: drop usb_buffer_alloc

This patch falls out of my work to fix usbmon so it uses virtual addresses.
It is not necessary, the "new" usbmon should work just fine with sisusbvga.
However, it seems ridiculous that anyone would use uncached memory to
transfer bulk data. Dropping the unnecessary use of usb_buffer_alloc
should be beneficial here, in case anyone ever uses the dongle on
anything beyond x86.

I had no success in raising the author of the driver by e-mail, so
the patch is not actually tested.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>