[ffmemless] Add missing function declaration

The ffmemless_erase_effect function declaration was missing
from the header file. Added so that the function can be used.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>
[ffmemless] Clear old effect before uploading new

If a effect with different type than an existing effect in kernel
is uploaded with same id, the effect loading fails. Solved this by
deleting the old effect before uploading new ones.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>
Merge pull request #24 from kjokinie/master

Clear old effect before uploading new