at91: Fix uhpck clock rate in upll case

The uhpck clock should be divided from the utmi clock, not its parent
(main). This change is mostly cosmetic as the uhpck rate value is not
used anywhere except for the debugfs clock output.

Signed-off-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>