char/serial: cosmetic fixes.

Some cosmetic fixes to char/serial fixing some checkpatch errors.

Cc: qemu-trivial@nongnu.org

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
