USB: host: ohci-at91: remove useless uclk clock

Now that the system clock driver is forwarding set_rate request to the
parent clock, we can safely call clk_set_rate on the system clk and get
rid of the uclk field.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>