Handle the case when /sys/class/tty/ entries are not symlinks.

Handle the case when /sys/class/tty/ entries are not symlinks.

For example:

$ uname -a
Linux RT-N66U 2.6.22.19 #1 Thu Feb 12 20:33:36 CST 2015 mips GNU/Linux

$ ls -lF /sys/class/tty/
drwxr-xr-x    2 foo  root             0 May 18 18:12 ttyACM0/

$ ls -lF /sys/class/tty/ttyACM0/
-r--r--r--    1 foo  root          4096 May 18 18:12 dev
lrwxrwxrwx    1 foo  root             0 May 18 18:12 device -> ../../../devices/pci0000:00/0000:00:04.1/usb1/1-1/1-1.1/1-1.1:1.0/
lrwxrwxrwx    1 foo  root             0 May 18 18:12 subsystem -> ../../../class/tty/
-rw-r--r--    1 foo  root          4096 May 18 18:12 uevent