uap8xxx: Use debugfs instead of file in procfs

Exposes debugfs entries for the data previously available at
"/proc/<interface>/debug", under "<debugfs>/uap8xxx/<interface>/".
As a bonus, multiple interfaces are properly handled; the old code
only worked okay for one interface due to the poor use of the
table describing the debug entries.
uap8xxx: Remove redefinition of the BIT() macro

The BIT() macro is defined by the kernel headers, there is no need to have
it repeated in the driver.