spi: Rename enable1 to activate in spi_set_cs()

The enable1 is confusing name. Change it to clearly show what is
the intention behind it. No functional changes.

Fixes: 25093bde

 ("spi: implement SW control for CS times")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210420131846.75983-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>