hw/pflash: refactor pflash_data_write()

hw/pflash: refactor pflash_data_write()

Move the offset calculation, do it once at the start of the function and
let the 'p' variable point directly to the memory location which should
be updated.  This makes it simpler to update other buffers than
pfl->storage in an upcoming patch.  No functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240108160900.104835-2-kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 3b14a555fdb627ac091559ef5931c887d06590d8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>