Allow setMISO/setMOSI/setSCK - after begin

[KurtE]

Made the SPI code work like the old stuff where you could set the miso/mosi/sck pins after the begin.

Some people actually used this for changing SPI on the fly to use multiple sets of pins

Tested with simple app on T3.6

#include <SPI.h>

void setup() {
  while (!Serial && (millis() < 3000)) ;
  Serial.begin(9600);
  Serial.println("SPI Test");

  SPI.begin();
  SPI.setMOSI(7);
  SPI.setMISO(8);
  SPI.setSCK(14);
}

void loop() {
  SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
  int errors = 0;
  for (int i=0; i < 10; i++) {
    int j = SPI.transfer(i);
    if (i != j) {
      Serial.printf("Mismatch %d != %d\n", i, j);
      errors = 1;
    }
  }
  if (errors == 0) 
    Serial.println("No Errors");
  SPI.endTransaction();
  delay(2000);

}

I verified without a jumper between 7 and 8 and looked at Logic analyzer and then put in jumper between 7 and 8.

[PaulStoffregen]

Thanks! :)