hwclock: fix --rtc option

hwclock: fix --rtc option

The --rtc option does not set the name of the device correctly.
It still uses /dev/rtc even if the --rtc option is given.

Testcase:
$ mv /dev/rtc /dev/foo
$ hwclock --show --debug --rtc=/dev/foo
hwclock from util-linux-2.13-rc2
Using /dev interface to clock.
Last drift adjustment done at 1190198135 seconds after 1969
Last calibration done at 1190198135 seconds after 1969
Hardware clock is on local time
Assuming hardware clock is kept in local time.
Waiting for clock tick...
hwclock: open() of /dev/rtc failed, errno=2: No such file or directory.
...got clock tick

Co-Author: Karel Zak <kzak@redhat.com>
Signed-off-by: Matthias Koenig <mkoenig@suse.de>
Signed-off-by: Karel Zak <kzak@redhat.com>