usbhost: USBHost::fileControlBuf() handle alignment and endianness

Based on great feedback from Martin Kojtal on my previous commit, I
have modified my USBHost::fileControlBuf() change to be more portable.
    https://github.com/adamgreen/mbed/commit/ddb3fbe826f800a62c68ae57b7deb7c4777527d8#commitcomment-3988044

The code now fills in the setupPacket byte array a byte at a time,
using bit shifts to extract lower and upper bytes from the 16-bit
values so that the code should work on big endian or little endian
machines.  I also removed the 2-byte alignment attribute from the
setupPacket array as it is no longer required.

Author: adamgreen

libraries/USBHost/USBHost/USBHost.cpp

@@ -1153,14 +1153,12 @@ USB_TYPE USBHost::controlTransfer(USBDeviceConnected * dev, uint8_t requestType,
 
 void USBHost::fillControlBuf(uint8_t requestType, uint8_t request, uint16_t value, uint16_t index, int len)
 {
-#ifdef __BIG_ENDIAN
-#error "Must implement BE to LE conv here"
-#endif
     setupPacket[0] = requestType;
     setupPacket[1] = request;
-    //We are in LE so it's fine
-    uint16_t* setupPacketHalfWords = (uint16_t*)setupPacket;
-    setupPacketHalfWords[1] = value;
-    setupPacketHalfWords[2] = index;
-    setupPacketHalfWords[3] = (uint32_t) len;
+    setupPacket[2] = (uint8_t) value;
+    setupPacket[3] = (uint8_t) (value >> 8);
+    setupPacket[4] = (uint8_t) index;
+    setupPacket[5] = (uint8_t) (index >> 8);
+    setupPacket[6] = (uint8_t) len;
+    setupPacket[7] = (uint8_t) (len >> 8);
 }

libraries/USBHost/USBHost/USBHost.h

@@ -252,7 +252,7 @@ class USBHost : public USBHALHost {
 #endif
 
     // to store a setup packet
-    __attribute__((aligned(2))) uint8_t  setupPacket[8];
+    uint8_t  setupPacket[8];
 
     typedef struct {
         uint8_t event_id;