m25p80: Improve command handling for Jedec commands

m25p80: Improve command handling for Jedec commands

When requesting JEDEC data using the JEDEC_READ command, the Linux kernel
always requests 6 bytes. The current implementation only returns three
bytes, and interprets the remaining three bytes as new commands.
While this does not matter most of the time, it is at the very least
confusing. To avoid the problem, always report up to 6 bytes of JEDEC
data. Fill remaining data with 0.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>