pwm: bcm2835: Drop open coded variant of devm_clk_rate_exclusive_get()

pwm: bcm2835: Drop open coded variant of devm_clk_rate_exclusive_get()

Since commit b0cde62e4c54 ("clk: Add a devm variant of
clk_rate_exclusive_get()") the clk subsystem provides
devm_clk_rate_exclusive_get(). Replace the open coded implementation by
the new function.

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/8e1a5151a7bcd455996c873bb3d13ab86def3490.1710078146.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>