boardd: smaller spi chunk size (#28360)

Co-authored-by: Comma Device <device@comma.ai>
old-commit-hash: 702e4120e6

selfdrive/boardd/spi.cc

@@ -163,7 +163,7 @@ int PandaSpiHandle::bulk_read(unsigned char endpoint, unsigned char* data, int l
 }
 
 int PandaSpiHandle::bulk_transfer(uint8_t endpoint, uint8_t *tx_data, uint16_t tx_len, uint8_t *rx_data, uint16_t rx_len, unsigned int timeout) {
-  const int xfer_size = 0x40 * 15;
+  const int xfer_size = 0x40 * 5;
 
   int ret = 0;
   uint16_t length = (tx_data != NULL) ? tx_len : rx_len;