staging: comedi: amplc_pc236: combine interrupt enable/disable functions

`pc236_intr_enable()` and `pc236_intr_disable()` are very similar.
Combine them into a single function `pc236_intr_update()` with a
parameter to indicate whether interrupts are being enabled or disabled.

Change type type of the `enable_irq` member of the private data to
`bool` to match the parameter.

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