Bluetooth: Audio: Remove zero-ing of variable

Commit dc812539 ("Bluetooth: MPL: Fix use of uninitialized
command.param value") fixed the use of a potentially un-initialized
variable, by
- splitting a log statement, so that the variable is only
  logged (=used) if in use (and therefore hopefully also initialized)
- setting the variable to zero if not in use

The second part is not required, as all other code in the module
checks whether the variable is in use before using it.  And this
second part also prevents the parent struct from being made 'const'.

This commits removes the zeroing of the variable, so that the struct
can be made 'const'.

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>