MIPS: DECstation I/O ASIC DMA interrupt classes

This change complements commits d0da7c002f7b2a93582187a9e3f73891a01d8ee4
[MIPS: DEC: Convert to new irq_chip functions] and
5359b938

 [MIPS: DECstation I/O ASIC DMA
interrupt handling fix] and implements automatic handling of the two
classes of DMA interrupts the I/O ASIC implements, informational and
errors.

Informational DMA interrupts do not stop the transfer and use the
`handle_edge_irq' handler that clears the request right away so that
another request may be recorded while the previous is being handled.

DMA error interrupts stop the transfer and require a corrective action
before DMA can be reenabled.  Therefore they use the `handle_fasteoi_irq'
handler that only clears the request on the way out.  Because MIPS
processor interrupt inputs, one of which the I/O ASIC's interrupt
controller is cascaded to, are level-triggered it is recommended that
error DMA interrupt action handlers are registered with the IRQF_ONESHOT
flag set so that they are run with the interrupt line masked.

This change removes the export of clear_ioasic_dma_irq that now does not
have to be called by device drivers to clear interrupts explicitly
anymore.  Originally these interrupts were cleared in the .end handler of
the `irq_chip' structure, before it was removed.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5874/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>