usb: phy: ab8500-usb: call phy_dis_work only when necessary

Modify ab8500_usb_set_peripheral() and ab8500_usb_set_host() code to
schedule phy_dis_work only when necessary in order to prevent regulator
count mismatch during reboot/shutdown.

Signed-off-by: Sakethram Bommisetti <sakethram.bommisetti@stericsson.com>
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>