usb: host: xhci-plat: Remove useless test before clk_disable_unprepare

clk_disable_unprepare() already checks that the clock pointer is valid.
No need to test it before calling it.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>