periph/eeprom: fix off-by-one error in debug statement

Since p is incremented DEBUG prints the next value instead of the current
I also took the liberty to add debugs to the `write` function

cpu/stm32/periph/eeprom.c

@@ -95,7 +95,7 @@ size_t eeprom_read(uint32_t pos, void *data, size_t len)
     for (size_t i = 0; i < len; i++) {
         _wait_for_pending_operations();
         *p++ = *(__IO uint8_t *)(EEPROM_START_ADDR + pos++);
-        DEBUG("0x%02X ", *p);
+        DEBUG("0x%02X ", *(p-1));
     }
     DEBUG("\n");
 
@@ -110,8 +110,10 @@ size_t eeprom_write(uint32_t pos, const void *data, size_t len)
 
     _unlock();
 
+    DEBUG("Writing data to EEPROM at pos %" PRIu32 ": ", pos);
     for (size_t i = 0; i < len; i++) {
         _write_byte((EEPROM_START_ADDR + pos++), *p++);
+        DEBUG("0x%02X ", *(p-1));
     }
 
     _lock();