mfd: omap-usb-tll: Clean up clock handling

Every channel has a functional clock that is similarly named.
It makes sense to use a for loop to manage these clocks as OMAPs
can come with up to 3 channels.

Dynamically allocate and get channel clocks depending on the
number of clocks avaiable on the platform.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>