pata_bf54x: fix BMIDE status register emulation

The author of this driver clearly wasn't familiar with the BMIDE specification
(also known as SFF-8038i) when he implemented the bmdma_status() method: first,
the interrupt bit of the BMIDE status register corresponds to nothing else but
INTRQ signal (ATAPI_DEV_INT here); second, the error bit is only set if the
controller encounters issue doing the bus master transfers, not on the IDE DMA
burst termination interrupts like here (moreover, setting the error bit doesn't
cause an interrupt).  We now need to disable all those unused interrupts...

(The only thing I couldn't figure out is how to flush the FIFO to memory once
the interrupt happens as required by the mentioned spec.)

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>