sunrpc: silence uninitialized variable warning

kstrtouint() can return a couple different error codes so the check for
"ret == -EINVAL" is wrong and static analysis tools correctly complain
that we can use "num" without initializing it.  It's not super harmful
because we check the bounds.  But it's also easy enough to fix.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>