There is only a single caller of pwm_sifive_enable() which only enables
or disables the clk. Put this implementation directly into
pwm_sifive_apply() which allows further simplification in the next
change.

There is no change in behaviour.
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: NEmil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>