Staging: hv: make get_channel_info static

This is done by moving it to the only .c file it is used in.

This also allowed us to delete the channel_interface.h file, as it's no
longer needed.

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>