Don't return mport from SyncReadMem.read(addr, en)

Change the read with enable method on SyncReadMem to not return the
CHIRRTL mport.  This results in saner code generation in CHIRRTL where
the mport can't be used in subsequent code blocks.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>