staging: comedi/drivers: Fixed sparse warnings

Fixed below sparse warnings in dt2811.c
 warning: symbol 'dt2811_clk_dividers' was not declared. Should it be static?
 warning: symbol 'dt2811_clk_multipliers' was not declared. Should it be static?

Signed-off-by: Nayeemahmed Badebade <itachi.opsrc@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>