esp-scsi: check for INTR_BS/INTR_FC instead of STAT_TC for command completion

esp-scsi: check for INTR_BS/INTR_FC instead of STAT_TC for command completion

The ESP SELATN command used to send SCSI commands from the ESP to the SCSI bus
is not a DMA command and therefore does not affect the STAT_TC bit. The only
reason this works at all is due to a bug in QEMU which (currently) always
updates the STAT_TC bit in ESP_RSTAT regardless of the state of the ESP_CMD_DMA
bit.

According to the NCR datasheet [1] the INTR_BS/INTR_FC bits are set when the
SELATN command has completed, so update the existing logic to check for these
bits in ESP_RINTR instead. Note that the read of ESP_RINTR needs to be
restricted to state == 0 as reading ESP_RINTR resets the ESP_RSTAT register
which breaks the STAT_TC check when state == 1.

This commit also includes an extra read of ESP_INTR to clear all the interrupt
bits before submitting the SELATN command to ensure that we don't accidentally
immediately progress to the data phase handling logic where ESP_RINTR bits have
already been set by a previous ESP command.

[1] "NCR 53C94, 53C95, 53C96 Advanced SCSI Controller"
    NCR_53C94_53C95_53C96_Data_Sheet_Feb90.pdf

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20230807065300.366070-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>