drivers: drop few redundant device_is_ready

Drop few redundant device_is_ready for functions that are only used as
argument to shell_device_filter, as shell_device_filter checks for that
alrady.

Suggested-by: Yishai Jaffe <yishai1999@gmail.com>
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>