[M487] Update spi_master_block_write() API

Author: ccli8

targets/TARGET_NUVOTON/TARGET_M480/spi_api.c

@@ -246,12 +246,12 @@ int spi_master_write(spi_t *obj, int value)
     return value2;
 }
 
-int spi_master_block_write(spi_t *obj, const char *tx_buffer, int tx_length, char *rx_buffer, int rx_length)
-{
+int spi_master_block_write(spi_t *obj, const char *tx_buffer, int tx_length,
+                           char *rx_buffer, int rx_length, char write_fill) {
     int total = (tx_length > rx_length) ? tx_length : rx_length;
 
     for (int i = 0; i < total; i++) {
-        char out = (i < tx_length) ? tx_buffer[i] : 0xff;
+        char out = (i < tx_length) ? tx_buffer[i] : write_fill;
         char in = spi_master_write(obj, out);
         if (i < rx_length) {
             rx_buffer[i] = in;