hw/block/onenand: Fix off-by-one error allowing out-of-bounds read

An off-by-one error in a switch case in onenand_read() allowed
a misbehaving guest to read off the end of a block of memory.

NB: the onenand device is used only by the "n800" and "n810"
machines, which are usable only with TCG, not KVM, so this is
not a security issue.

Reported-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20181115143535.5885-2-peter.maydell@linaro.org
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>