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.