hw/onenand: Remove unnecessary argument from onenand_command()

hw/onenand: Remove unnecessary argument from onenand_command()

Refactor onenand_command() -- since it is essentially a method of
the device object, it doesn't make sense to pass in something as
an argument which is one of the object's own member fields.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
[Riku Voipio: Fixes and restructuring patchset]
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
[Peter Maydell: More fixes and cleanups for upstream submission]
Signed-off-by:  Peter Maydell <peter.maydell@linaro.org>