staging: comedi: cb_das16_cs: remove unneeded default case in analog in read

The default (return -EINVAL) case is not needed when working
out the correct value to set the analog input range. As pointed
out by Ian Abbott, the comedi core checks the range in
comedi_check_chanlist() before calling the insn_read() function.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>