Change references to serial_hds[] to serial_hd()

Change all the uses of serial_hds[] to go via the new
serial_hd() function. Code change produced with:
 find hw -name '*.[ch]' | xargs sed -i -e 's/serial_hds\[\([^]]*\)\]/serial_hd(\1)/g'

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180420145249.32435-8-peter.maydell@linaro.org