sound: oss: Use kernel_read_file_from_path() for mod_firmware_load()

Since recently we have kernel_read_file_from_path(), and it's doing
the same thing as our own home-baked mod_firmware_load().  Let's use
the official API function and clean up the old code.

Signed-off-by: Takashi Iwai <tiwai@suse.de>