usb: ohci-da8xx: let the regulator framework keep track of use count

There's no reason to have a separate variable to keep track of the
regulator state. The regulator core already does that. Remove
reg_enabled from struct da8xx_ohci_hcd.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>