Cannot select devices by gpu_device.

[Masa-tam]

Since 1.7.3, whisper.cpp uses the first device in the device list without looking at the value of gpu_device.
I was able to fix it with the following modification, but I am not sure if it is correct or not.

First, take a count in “static ggml_backend_t whisper_backend_init_gpu(const whisper_context_params & params)” and select a device.
Then, in “static ggml_backend_buffer_type_t whisper_default_buffer_type(const whisper_context_params & params)”, the device is selected by the same count.