staging: comedi: ni_labpc: return error from labpc_counter_set_mode()

Make sure labpc_counter_set_mode() was successfull and return any
errno back to the caller.

For the if/else conditions that either load the counter of just set
the mode, use a common error patch to check for the error.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>