hw/block/pflash_cfi02: Add helpers to manipulate the status bits

hw/block/pflash_cfi02: Add helpers to manipulate the status bits

Pull out all of the code to modify the status into simple helper
functions. Status handling becomes more complex once multiple
chips are interleaved to produce a single device.

No change in functionality is intended with this commit.

Signed-off-by: Stephen Checkoway <stephen.checkoway@oberlin.edu>
Message-Id: <20190426162624.55977-3-stephen.checkoway@oberlin.edu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Extracted from bigger patch]
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>