core: Make event_data_lock non-recursive

core: Make event_data_lock non-recursive

Hans pointed out that usbi_fd_notification() is only used by the
functions that add/remove pollfds. They already hold the required
lock, so to make it less expensive we will assume the lock is
already held.

The usbi_fd_notification() function has also been moved from core.c
into io.c and made static, since it now only has one use case.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>