pwm: fix subscription error

tmp_str array has a size of 1, so tmp_str[1] access is out of range.
Besides file_read_str() reads only one byte and strncmp() compares only
one byte too.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Merge pull request #3 from yegorich/buf

pwm: fix buffer overflow error in get polarity