pflash_cfi01: Log use of flawed "write to buffer"

pflash_cfi01: Log use of flawed "write to buffer"

Our implementation of "write to buffer" (command 0xE8) is flawed.
LOG_UNIMP its use, and add some FIXME comments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20190308094610.21210-4-armbru@redhat.com>
(cherry picked from commit 4dbda935e054600667f9e57095fa97e2ce5936f9)
*prereq for 3a283507
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>