m25p80: Improve command handling for unsupported commands

m25p80: Improve command handling for unsupported commands

Whenever an unsupported command is encountered, the current code
interprets each transferred byte as new command. Most of the time, those
'commands' are interpreted as new unknown commands. However, in rare
cases, it may be that for example address or length information
passed with the original command is by itself a valid command.
If that happens, the state machine may get completely confused and,
worst case, start writing data into the flash or even erase it.

To avoid the problem, transition into STATE_READING_DATA and keep
sending a value of 0 until the chip is deselected after encountering
an unsupported command.

Signed-off-by: Guenter Roeck <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>