remoteproc: fix the error check for idr_alloc

The new idr_alloc interface returns the allocated id back
on success, so fix the error path to check for negative
values. This was missed out in the newer idr interface
adoption patch, 15fc6110 "remoteproc: convert to idr_alloc()".

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>