rombios/ata: Do not wait for BSY to be set

rombios/ata: Do not wait for BSY to be set

After issuing a reset, the BSY bit is expected to be set.  This is not the
case for Qemu.

In SeaBIOS.git: 580e33293244fee4556e56ecc67b8bd877f3c496

this check was even replaced with a udelay(5), as enough real hardware ignored
the BSY bit as well.

As rombios does not have an equivalent udelay(), replace the wait with a write
to port 0x80 which is whitelisted by Xen for 'a small delay'.

This causes 42k fewer IO traps to Qemu.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>