usb: pxa27x_udc: remove IRQ_USB define

IRQ_USB was implicitly included by gpio.h. Use the existing variable
instead of IRQ_USB.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>