USB: at91_udc: Additional checks

This patch performs additional checks in at91_udc, just in case of
some spurious interrupts or device enumeration.

Signed-off-by: Wojtek Kaniewski <wojtekka@toxygen.net>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>