habanalabs: add missing error check in sysfs max_power_show

Add a missing error check in the sysfs show function for max_power.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>