We used to put the device name without the "/dev/" component into
device_detect_by_name() and cdev_by_name(). Since 8ca7053b3ff we only
put the device name into device_detect_by_name(), but cdev_by_name()
still gets the full path. The result is that a "mount /dev/<devname>" no
longer works. Fix this.

Fixes: 8ca7053b3ff ("commands: mount: Make use of devpath_to_name()")

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Andrey Smirnov <andrew.smirnov@gmail.com>