drivers: aio: remove aio disabling before invoking callback

Remove aio disabling code before calling callback. If aio is
disabled before callback is invoked, calling pending
interrupt check API in the callback will get negative result.
If user wants to disable the aio, it can do this in callback
instead of this being done blindly before callback.

Jira: ZEP-1437

Change-Id: I42ebe4584af6396ac2360152f9b0e2c389c44145
Signed-off-by: Baohong Liu <baohong.liu@intel.com>