usb: phy: am335x-control: include appropriate header file in phy-am335x-control.c

Include header file drivers/usb/phy/am35x-phy-control.h in
phy/phy-am335x-control.c because function am335x_get_phy_control() has
its prototype declaration in drivers/usb/phy/am35x-phy-control.h.

Also, remove definition of structure phy_control because it is already
defined in the included header.

This eliminates the following warning in phy/phy-am335x-control.c:
drivers/usb/phy/phy-am335x-control.c:114:21: warning: no previous prototype for ‘am335x_get_phy_control’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>