pwm: sifive: Fold pwm_sifive_enable() into its only caller

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: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>