usb: host: tegra: don't touch EMC clock

Clock "emc" is for the External Memory Controller. The USB driver has no
business touching this clock directly. Remove the code that does so.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Stephen Warren <swarren@nvidia.com>