serial: Change the wait for carrier locking

We want to push the lock/unlock into the helper functions so that we
can prepare to move to using the tty_port helper. The expansion initially
comes out a bit ugly but its worth the temporary expansion IMHO just so
we can produce a nice testable series of changes.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>