mfd: Fix usbhs_enable error handling

In the case of missing platform_data we do not hold a spin_lock,
thus we should not call spin_unlock_irqrestore in the error path.

Also simplify the error handling by separating the successful path
from error path. I think this change improves readability.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Tested-by: Steve Calfee <stevecalfee@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>