usb: Missing dma_mask in ehci-vt8500.c when probed from device-tree

Device-tree probed devices don't get a dev.dma_mask set. This patch
sets a default 32bit mask on arch-vt8500 when using devicetree.

Without this patch, arch-vt8500 cannot detect ehci attached devices.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>