fs/char_dev.c: chrdev_open marked static and removed from fs.h

There is an outdated comment in serial_core.c also fixed.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>