MIPS: DECstation I/O ASIC DMA interrupt handling fix

MIPS: DECstation I/O ASIC DMA interrupt handling fix

This change complements commit d0da7c002f7b2a93582187a9e3f73891a01d8ee4
and brings clear_ioasic_irq back, renaming it to clear_ioasic_dma_irq at
the same time, to make I/O ASIC DMA interrupts functional.

Unlike ordinary I/O ASIC interrupts DMA interrupts need to be deasserted
by software by writing 0 to the respective bit in I/O ASIC's System
Interrupt Register (SIR), similarly to how CP0.Cause.IP0 and CP0.Cause.IP1
bits are handled in the CPU (the difference is SIR DMA interrupt bits are
R/W0C so there's no need for an RMW cycle).  Otherwise the handler is
reentered over and over again.

The only current user is the DEC LANCE Ethernet driver and its extremely
uncommon DMA memory error handler that does not care when exactly the
interrupt is cleared.  Anticipating the use of DMA interrupts by the Zilog
SCC driver this change however exports clear_ioasic_dma_irq for device
drivers to choose the right application-specific sequence to clear the
request explicitly rather than calling it implicitly in the .irq_eoi
handler of `struct irq_chip'.  Previously these interrupts were cleared in
the .end handler of the said 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/5826/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>