staging: comedi: ni_daq_dio24: add back missing 'if (ret)'

The 'if (ret)' after calling comedi_pcmcia_enable() was accidentally
removed in:

Commit: 573a9648
staging: comedi: ni_daq_dio24: use comedi_pcmcia_{enable,disable}

Put if back so that dio24_auto_attach() can finish attaching to
the board after enabling the pcmcia device.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>