staging: comedi: ni_labpc: rename subdevice (*insn_{read, write}) functions

For aesthetic reasons, and to make grepping easier, rename all the
subdevice (*insn_read) and (*insn_write) functions.

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