spi: dw: Make debugfs name unique between instances

[ Upstream commit 13288bdf ]

Some system have multiple dw devices. Currently the driver uses a
fixed name for the debugfs dir. Append dev name to the debugfs dir
name to make it unique.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>