slim-msm-ngd: Do not enable/disable core irq in SSR/PDR notifications

During SSR testing, subsystem and qmi notifications for SSR/PDR
sequence is missing sometimes. We are seeing 2 times DOWN notification
without UP. This will lead to imbalance in disabling and enabling of
core IRQ and capability exchange is timing out.

If interrupt line is not enabled, during retry-capability exchange
request is timing out and leading to freed memory access
inside ngd_xfer_msg.

Fix this issue by removing enabling and disabling from ngd_slim_enable
enable it only once during probe.

Change-Id: I15616c6e284848ed1f2aca00d251b82f79feb803
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>