comedi: comedi_8254: correct kernel-doc warnings

comedi: comedi_8254: correct kernel-doc warnings

Correct typos in 3 struct member descriptions to eliminate all
kernel-doc warnings in comedi_8254.h:

Warning: include/linux/comedi/comedi_8254.h:112 struct member 'next_div'
 not described in 'comedi_8254'
Warning: include/linux/comedi/comedi_8254.h:112 struct member 'clock_src'
 not described in 'comedi_8254'
Warning: include/linux/comedi/comedi_8254.h:112 struct member 'gate_src'
 not described in 'comedi_8254'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20251214202727.2215461-1-rdunlap@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>