hw/sd/bcm2835_sdhost: Fix PIO mode writes

hw/sd/bcm2835_sdhost: Fix PIO mode writes

Writes in PIO mode have two requirements:

- A data interrupt must be generated after a write command has been
  issued to indicate that the chip is ready to receive data.
- A block interrupt must be generated after each block to indicate
  that the chip is ready to receive the next data block.

Rearrange the code to make this happen. Tested on raspi3 (in PIO mode)
and raspi2 (in DMA mode).

Signed-off-by: Guenter Roeck <[email protected]>
Message-id: 1531779837[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>