BQLmon: rework open/read/close

There is no need to open/read/close the sysfs attribute file, rather,
let's just rewind to the beginning of the file once we read its content.

While at it, we can remove a bunch of now unused strings storing paths
we just need to access once during initialization.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
BQLmon: ignore perf.data* files

Running "perf" in the same directory is not uncommon, so let's just
ignore these files.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>