Revert "cdc_acm : Restrict writing more than 4 bytes into TX USB Endpoint."

This reverts commit 1da0a9ee.

The workaround caused a severe performance penalty, and only worked
for USB packets of 4-15 bytes in length (16+ byte packets weren't
subject to the hardware bug).  Single-byte packets (very common for
cdc_acm serial port transfers) would still be duplicated sometimes.

The upcoming DMA implementation does not share the performance
penalty, and also is not subject to the bug for those sizes of packets
(though it DOES still have a problem with single-byte packets!).

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>