firmware: use static inlines for state machine checking

This will allow us to do proper typechecking on users both of
values passed and return types expected.

While at it, change the parameter passed to be the struct fw_priv,
so we can move around the state machine variable as we see fit with
these helpers.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>