tf700t|tf300t: libhealthd: fix pluggable status

The board only should report a pluggable status if and only if the status sysfs
reports a valid charging status

Change-Id: I4d71ca280d7d459a1ae15774bd264c228d9d5056
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge pull request #4 from CyanogenMod/cm-11.0

tf700t|tf300t: libhealthd: fix pluggable status