spi: dw: eliminate speed variable in ->transfer_one()

There is no point to have a separate variable for speed in ->transfer_one().

While here, remove !chip->clk_div from a condition since it is assigned
simultaneously with chip->speed_hz. We can do this safely because
 a) transfer speed can't be higher than max_freq and therefore chip->clk_div
    can be 0 only when chip->speed_hz is 0, and
 b) transfer speed can't be 0, otherwise we will get division by zero
    exception.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>