pwm: fix unclosed file descriptors on free
specify that pwm subsystem is only support from 3.12 onwards
document change to debug-code default
README: fix errors
gpio: remove check for MAX_GPIO_ID

Remove this check, as max GPIO number can vary from platform to
platform.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Merge pull request #4 from yegorich/gpio_num

gpio: remove check for MAX_GPIO_ID