pwm: rockchip: Replace "bus clk" with "PWM clk"

Clarify the Rockchip PWM driver's error messages by referring to the clock
that operates a PWM device as the "PWM" clock, matching its name in the
device tree, rather than the "bus" clock (which is especially misleading in
the case of devices that also use a separate clock for bus access).

Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Simon South <simon@simonsouth.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>