spi: More refacroings after multi-CS support

Merge series from Andy Shevchenko <andriy.shevchenko@linux.intel.com>:

A couple of additional refactorings on top of the multi-CS support.
One is to make sure that the comment and the code are not disrupted
if additional changes come in the future and second one is f or the
sake of deduplication. In both cases it also makes indentation level
smaller in the affected pieces of the code.

No functional changes intended.