Input: adp5589-keys - use device-managed function in adp5589_keypad_add()

Input: adp5589-keys - use device-managed function in adp5589_keypad_add()

This change makes use of the devm_input_allocate_device() function, which
gets rid of the input_free_device() and input_unregister_device() calls.

When a device is allocated via input_allocate_device(), the
input_register_device() call will also be device-managed, so there is no
longer need to manually call unregister.

Also, the irq is allocated with the devm_request_threaded_irq(), so with
these two changes, the adp5589_keypad_remove() function is no longer
needed.

This cleans up the error & exit paths.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201112074308.71351-3-alexandru.ardelean@analog.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>