remoteproc: calculate max_notifyid by counting vrings

Simplify handling of max_notifyid by simply counting the
number of vrings.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Acked-by: Ido Yariv <ido@wizery.com>
[small terminology changes]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>