usb: phy: ab8500-usb: split ab8500_usb_phy_ctrl

Split ab8500_usb_phy_ctrl into separate enable/disable functions to make
the code more linear and readable.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>