firmware: microchip: fail auto-update probe if no flash found

firmware: microchip: fail auto-update probe if no flash found

There's no point letting the driver probe if there is no flash, as
trying to do a firmware upload will fail. Move the code that attempts
to get the flash from firmware upload to probe, and let it emit a
message to users stating why auto-update is not supported.
The code currently could have a problem if there's a flash in
devicetree, but the system controller driver fails to get a pointer to
it from the mtd subsystem, which will cause
mpfs_sys_controller_get_flash() to return an error. Check for errors and
null, instead of just null, in the new clause.

CC: stable@vger.kernel.org
Fixes: ec5b0f1193ad4 ("firmware: microchip: add PolarFire SoC Auto Update support")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>