USB: atmel_usba_udc: fix freeing irq in usba_udc_remove()

Add a free_irq() call on vbus gpio when we remove udc so that the
vbus irq is properly released.

Signed-off-by: Rob Emanuele <rje@crystalfontz.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>