usb: phy: mv-usb: remove incorrect __exit_p annotation

When platform_driver_probe() is not used, bind/unbind via sysfs
is enabled.  Thus, __exit_p annotation should be removed from
remove().

Also, mv_otg_remove() is staticized, because this function is
used only in this file. Fix the following sparse warning:

drivers/usb/phy/phy-mv-usb.c:656:5: warning: symbol 'mv_otg_remove' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>