uio: uio_hv_generic: use devm_kzalloc() for private data alloc

uio: uio_hv_generic: use devm_kzalloc() for private data alloc

This is a minor cleanup for the management of the private object of this
driver. The allocation can be tied to the life-time of the hv_device
object.
This cleans up a bit the exit & error paths, since the object doesn't need
to be explicitly free'd anymore.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201119154903.82099-4-alexandru.ardelean@analog.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>