usb: ohci-s3c2410.c: Use clk_prepare_enable/clk_disable_unprepare

This patch modifies the ohci-s3c2410 driver to prepare and unprepare
clocks in addition to enabling and disabling, since it is required
by common clock framework.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>