staging: emxx_udc: Added missing __iomem modifier to handle p_regs

Since in nbu2ss_drv_probe() p_regs is assigned from mmio_base, which is
marked as __iomem, p_regs also should be market with __iomem.

Signed-off-by: Carmeli Tamir <carmeli.tamir@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>