firmware: cs_dsp: Cleanup debugfs for wmfw and bin

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

These two patches improve the implementation of the debugfs files for
the wmfw and bin file names. First patch removes duplicated code. Second
patch replaces the old clunkiness of storing the filename with an appended
\n. The \n can be appended when the file is read, to keep the stored string
sane.