staging: comedi: ni_labpc: pass ai scan mode through various functions

Pass the `enum scan_mode` value calculated by `labpc_ai_scan_mode()` as
a parameter to various other functions so they don't have to call it
themselves.  Amongst others, the affected functions include
`labpc_adc_timing()`, `labpc_ai_convert_period()` and
`labpc_ai_scan_period()`.

`labpc_adc_timing()` calls `labpc_ai_convert_period()` and
`labpc_ai_scan_period()` in several places, but the returned values are
the same each time, so change it to just call those functions once and
re-use the return values.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>