staging: comedi: amplc_pci230: simplify interrupt enable handling

`struct pci230_private` has two members to manage the enabled interrupt
sources.  `int_en` is the interrupt sources we want to be enabled and
`ier` is a shadow of the write-only interrupt enable register.  They
have the same value most of the time.  They differ in the interrupt
handler (`pci230_interrupt()`) itself when it temporarily clears bits in
the interrupt enable register and the `ier` member in order to unlatch
them in hardware, but leaves the `int_en` member alone.  They also
differ in `pci230_ai_stop()` and `pci230_ao_stop()` which clear bits in
the `int_en` member and wait for the interrupt handler to finish before
copying the value to the `ier` member and the interrupt enable register.

Simplify the handling a bit, by making the `ier` member take on the role
of the `int_en` member, and allowing the value to differ from the
interrupt enable register while the interrupt handler is running.

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