remoteproc: fix off-by-one bug in __rproc_free_vrings

Fix a nasty off-by-one bug in __rproc_free_vrings which
resulted in a memory leak and (for some platforms) failures
to reload the remote processor.

Signed-off-by: Subramaniam Chanderashekarapuram <subramaniam.ca@ti.com>
[ohad@wizery.com: reword commit log, stick with the for loop]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>