usb: phy: twl4030-usb: Use mutex instead of spinlock for protecting the data

We're using threaded irq on a I2C bus and we're sleeping in
twl4030_usb_irq() as it calls twl4030_usb_linkstat() which
calls the i2c functions. If we ever need to lock for longer
I2C transaction sequences a mutex will allow us to do that
easily.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>