USB: pxa27x_udc: avoid compiler warnings and misbehavior on buggy hardware

hardware reports wrong interrupt.  Although such a situation should not
happen, the compiler complains about this access.

This patch adds a sanity check and generates warning to detect such
issues.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>