🐛 Fix SDSUPPORT for SKR CR-6 (#22668)

Co-authored-by: Sebastiaan Dammann <sebastiaandammann@outlook.com>

Marlin/src/pins/stm32f1/pins_BTT_SKR_CR6.h

 #if SD_CONNECTION_IS(ONBOARD)
   #define SD_DETECT_PIN                     PC4
   #define ONBOARD_SD_CS_PIN                 PA4   // Chip select for "System" SD card
+  #define SDSS                              ONBOARD_SD_CS_PIN
 #endif
 
 //