pwm: brcmstb: Remove useless locking

The lock only protects against concurrent users of the PWM API. This is not
expected to be necessary. And if there was such an issue, this is better
handled in the PWM core instead as it affects all drivers in the same way.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>