USB: re-remove <linux/usb_sl811.h>

Remove <linux/usb_sl811.h> ... somehow this was recreated when
the Blackfin arch was merged, instead of using <linux/usb/sl811.h>
which is the correct header.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>