spi: dw: Fix checkpatch issue

Fix the following checkpatch warnings.

  WARNING: debugfs_remove_recursive(NULL) is safe this check is probably not required
  WARNING: min() should probably be min_t(u32, rx_left, dw_readw(dws, DW_SPI_RXFLR))
  WARNING: else is not generally useful after a break or return
  WARNING: Missing a blank line after declarations
  WARNING: void function return statements are not generally useful

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>