staging: tidspbridge: remove unused header

No functional changes.

The header file drv_interface.h was only used locally, hence there's no need
to have it.

Also the only prototyped functions were the file_operations callbacks, then
this commit moves them up to avoid prototyping too.

Signed-off-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>