Staging: comedi: Fix code style in jr3_pci.c

Static variables are initialised to 0 by GCC.
Fixes checkpatch.pl error:
  ERROR: do not initialise statics to 0 or NULL
  #684: FILE: jr3_pci.c:684:
  +	static const struct jr3_pci_board *board = NULL;

Signed-off-by: Vladimir A. Nazarenko <naszar@ya.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>