cnss: Add cnss_get_restart_level() support

WLAN host driver requires this API to get SSR state, avoid doing
ramdump when SSR happen and quick restart WLAN subsys.

Add back subsys_get_restart_level() support and fix return -EIVAL
in cnss_put_hw_resources().

Change-Id: I902d81b5693ab76028ed7e97183e07ea970930c1
CRs-Fixed: 2945893
Signed-off-by: Will Huang <wilhuang@codeaurora.org>