spi: dw: Make debugfs use bus num and make irq name unique

Instead of using device name it was suggested that bus number was more
appropriate to differentiate debugfs names. Also reduce buffer size to
more realistic 32 bytes instead of 128.

When request_irq is called the bus number may not be assigned. Therefore
the irq name was not unique when dynamic bus number was being used.
As per most of the spi drivers use the device name instead. No other
use of dws->name could be found so it was removed.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Mark Brown <broonie@kernel.org>