staging: comedi: s526: wait for analog output conversions to complete

It's possible for the user to write more than one value to an analog
output channel with the (*insn_write) operation. Use comedi_timeout()
to check the interrupt status register to ensure that each data
conversion is complete before writing the next value.

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