Fix ESP8266 SPI divisor calculation

SdFat/src/SdSpiCard/SdSpi.h

    */
   void beginTransaction(uint8_t divisor) {
 #if ENABLE_SPI_TRANSACTIONS
-    SPISettings settings(F_CPU/(divisor ? divisor : 1), MSBFIRST, SPI_MODE0);
-    SPI.beginTransaction(settings);
+    SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
 #else  // ENABLE_SPI_TRANSACTIONS
     SPI.setBitOrder(MSBFIRST);
     SPI.setDataMode(SPI_MODE0);
+#endif  // ENABLE_SPI_TRANSACTIONS    
 #ifndef SPI_CLOCK_DIV128
     SPI.setClockDivider(divisor);
 #else  // SPI_CLOCK_DIV128
     }
     SPI.setClockDivider(v);
 #endif  // SPI_CLOCK_DIV128
-#endif  // ENABLE_SPI_TRANSACTIONS
   }
   /**
    * End SPI transaction.

SdFat/src/SdSpiCard/SdSpiESP8266.cpp

 //------------------------------------------------------------------------------
 /** Set SPI options for access to SD/SDHC cards.
  *
- * \param[in] divisor SCK clock divider relative to the system clock.
+ * \param[in] divisor SCK clock divider relative to the max SPI clock.
  */
 void SdSpi::beginTransaction(uint8_t divisor) {
+  const uint32_t F_SPI_MAX = 80000000;
 #if ENABLE_SPI_TRANSACTIONS
   // Note: ESP8266 beginTransaction does not protect for interrupts.
-  SPISettings settings(F_CPU/(divisor ? divisor : 1), MSBFIRST, SPI_MODE0);
+  SPISettings settings(F_SPI_MAX/(divisor ? divisor : 1), MSBFIRST, SPI_MODE0);
   SPI.beginTransaction(settings);
 #else  // ENABLE_SPI_TRANSACTIONS
   // Note: ESP8266 beginTransaction is the same as following code.
-  SPI.setFrequency(F_CPU/(divisor ? divisor : 1));
+  SPI.setFrequency(F_SPI_MAX/(divisor ? divisor : 1));
   SPI.setBitOrder(MSBFIRST);
   SPI.setDataMode(SPI_MODE0);
 #endif  // ENABLE_SPI_TRANSACTIONS