fix MT6701 sensor on RP2040

SPI on RP2040 seems to read an extra bit from MT6701, which STM32 does
not seem to read.

Author: Richard Unger

src/encoders/mt6701/MagneticSensorMT6701SSI.cpp

@@ -38,5 +38,5 @@ uint16_t MagneticSensorMT6701SSI::readRawAngleSSI() {
     spi->endTransaction();
     if (nCS >= 0)
         digitalWrite(nCS, HIGH);
-    return (value>>2)&0x3FFF;
+    return (value>>MT6701_DATA_POS)&0x3FFF;
 };

src/encoders/mt6701/MagneticSensorMT6701SSI.h

@@ -9,6 +9,11 @@
 
 #define MT6701_BITORDER MSBFIRST
 
+#if defined(TARGET_RP2040)
+#define MT6701_DATA_POS 1
+#else
+#define MT6701_DATA_POS 2
+#endif
 
 // Use SPI mode 2, capture on falling edge. First bit is not valid data, so have to read 25 bits to get a full SSI frame.
 // SSI frame is 1 bit ignore, 14 bits angle, 4 bit status and 6 bit CRC.