clk: vt8500: Fix "fix device clock divisor calculations"

Patch 72480014b8 "Fix device clock divisor calculations" was apparently
rebased incorrectly before it got upstream, causing a build error.

Replacing the "prate" pointer with the local parent_rate is most
likely the correct solution.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Tony Prisk <linux@prisktech.co.nz>
Cc: Mike Turquette <mturquette@linaro.org>