usb: ehci: Move common mxs code to separate functions (ehci_hcd_{stop|start})

Those functions will be re-used when the ehci MXS driver (for imx28)
will be converted to also support CONFIG_DM_USB.

No functional changes introduced - only cosmetic changes (u32 type)
and alignment to pass checkpatch.

Signed-off-by: Lukasz Majewski <lukma@denx.de>