Staging: comedi: ni_65xx.c: fix insn_bits shift calculation.

Fix insn_bits bitshift calculation for subdevice with non-zero
base_port.

Thanks to cJ-comedi at zougloub dot eu for spotting the bug.

Signed-off-by: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>