staging: comedi: s526: cleanup s526_gpct_insn_config()

Use a local variable for the iobase of the channel being configured.
This makes the outw() calls a bit cleaner.

Remove the unnecessary casting of the values being written to
the channel registers.

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>