pwm: sifive: Simplify using devm_clk_get_prepared()

Instead of preparing the clk after it was requested and unpreparing in
.probe()'s error path and .remove(), use devm_clk_get_prepared() which
copes for unpreparing automatically.

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