rombios/ata: wait for BSY to clear after write

rombios/ata: wait for BSY to clear after write

After rombios transfers the data for a write, it checks the status and
fails if BSY is set. qemu-trad doesn't set BSY for PIO writes, but QEMU
upstream does, and this causes rombios to fail writes because they are
marked as BSY. Instead, wait for BSY to clear after a write.

INT 13 writes are probably rarely used these days, but they are used by
GRUB 2 to write to its environment file which happens by default on
Ubuntu.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>