usb: gadget: f_fs: pull out f->disable() from ffs_func_set_alt()

usb: gadget: f_fs: pull out f->disable() from ffs_func_set_alt()

The ``alt`` parameter was used as a way to differentiate between
f->disable() and f->set_alt(). As the code paths diverge quite a bit,
pull out the f->disable() code from ffs_func_set_alt(), everything will
become clearer and less error prone. No change in functionality
intended.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20240802140428.2000312-3-tudor.ambarus@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>