Bluetooth: btmrvl: use native helpers for debugfs

Clean up the code by using native debugfs helpers, instead of implementing
them ourselves:

	debugfs_create_u8()
	debugfs_create_x16()
	debugfs_create_file()
	debugfs_remove_recursive()

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>