drivers: spi: xmc4xxx: Fix compiler warning with debug flags enabled

Fixes warning when CONFIG_DEBUG_OPTIMIZATION=y:

zephyrproject/modules/hal/infineon/XMCLib/drivers/inc/xmc_usic.h:2132:18:
warning: 'clock_settings' may be used uninitialized [-Wmaybe-uninitialized]
 2132 |                  (uint32_t)passive_level |
      |                  ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>