SD: avoid timeout lock if millis() approaches 2^16

this is a rework of @Timmmm 's PR arduino#1977

Author: facchinm

libraries/SD/src/utility/Sd2Card.cpp

@@ -283,7 +283,7 @@ uint8_t Sd2Card::init(uint8_t sckRateID, uint8_t chipSelectPin) {
 
   // command to go idle in SPI mode
   while ((status_ = cardCommand(CMD0, 0)) != R1_IDLE_STATE) {
-    if (((uint16_t)millis() - t0) > SD_INIT_TIMEOUT) {
+    if (((uint16_t)(millis() - t0)) > SD_INIT_TIMEOUT) {
       error(SD_CARD_ERROR_CMD0);
       goto fail;
     }
@@ -305,7 +305,7 @@ uint8_t Sd2Card::init(uint8_t sckRateID, uint8_t chipSelectPin) {
 
   while ((status_ = cardAcmd(ACMD41, arg)) != R1_READY_STATE) {
     // check for timeout
-    if (((uint16_t)millis() - t0) > SD_INIT_TIMEOUT) {
+    if (((uint16_t)(millis() - t0)) > SD_INIT_TIMEOUT) {
       error(SD_CARD_ERROR_ACMD41);
       goto fail;
     }