smbus: do not immediately complete commands

PIIX4 errata says that "immediate polling of the Host Status Register BUSY
bit may indicate that the SMBus is NOT busy."
Due to this, some code does the following steps:
(a) set parameters
(b) start command
(c) check for smbus busy bit set (to know that command started)
(d) check for smbus busy bit not set (to know that command finished)

Let (c) happen, by immediately setting the busy bit, and really executing
the command when status register has been read once.

This fixes a problem with AMIBIOS, which can now properly initialize the PIIX4.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
