cnss2: Clear device error flag at the end of shutdown process

During driver unload or idle shutdown, device error may also happen so
in order to get firmware dump, clear the device error flag at the end
of shutdown process.

Change-Id: If8e96cda2068fe2050aef73da356b51f62805adb
Signed-off-by: Yue Ma <yuem@codeaurora.org>