Bluetooth: introduce DEFINE_SHOW_ATTRIBUTE() macro

This macro deduplicates a lot of similar code across the hci_debugfs.c
module. Targeting to be moved to seq_file.h eventually.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>