Fix MSP flash read so it returns the actual bytes read from the flash,

and not the amount of bytes requested, which may be different depending
on the constraints of the flash chip used.

src/main/interface/msp.c

@@ -375,6 +375,8 @@ static void serializeDataflashReadReply(sbuf_t *dst, uint32_t address, const uin
 #endif
 
     if (compressionMethod == NO_COMPRESSION) {
+
+        uint16_t *readLenPtr = (uint16_t *)sbufPtr(dst);
         if (!useLegacyFormat) {
             // new format supports variable read lengths
             sbufWriteU16(dst, readLen);
@@ -383,6 +385,11 @@ static void serializeDataflashReadReply(sbuf_t *dst, uint32_t address, const uin
 
         const int bytesRead = flashfsReadAbs(address, sbufPtr(dst), readLen);
 
+        if (!useLegacyFormat) {
+            // update the 'read length' with the actual amount read from flash.
+            *readLenPtr = (uint16_t)bytesRead;
+        }
+
         sbufAdvance(dst, bytesRead);
 
         if (useLegacyFormat) {