Change references to serial_hds[] to serial_hd()

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 <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-id: 20180420145249[email protected]