net: ti: icss-iep: Remove spinlock-based synchronization

As all sources of concurrency in hardware register access occur in
non-interrupt context eliminate spinlock-based synchronization and
rely on the mutex-based synchronization that is already present.

Signed-off-by: Diogo Ivo <diogo.ivo@siemens.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>