firmware: cirrus: cs_dsp: Remove non-existent member from kerneldoc

The kerneldoc for struct cs_dsp refers to a fw_file_name member but
there's no such member.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://msgid.link/r/20240307105516.40250-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>