staging: comedi: s526: cleanup s526_gpct_rinsn()

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

Move the masking of the read data to make the value stored in
the data array a bit clearer.

The comedi core expects insn_read functions to return the number
of insn data values read. For this function, the final value of
'i' is correct but change the return to 'insn->n' just to make
it clear.

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>