spi: dw: refactor code that handles clk_div

This patch does the following changes:

a) the calculation of clk_div is simplified to oneliner;

b) chip->clk_div is updated if clk_div is not zero, therefore the condition is
   simplified by using chip->clk_div in both cases;

c) while here, the redundant parentheses are removed.

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