UPSTREAM: usb: phy: msm: switch over to writel()

Remove that single instance of writel_relaxed()
call which is only available on ARM architecture.

This will let us build test this driver on all
different architectures.

Reviewed-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>