nfp: move mutex init out of net code

Move mutex init to main file close to structure allocation.
This will allow mutex to be taken before net code runs (e.g.
from devlink callbacks).  While at it remember to destroy
the mutex.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>