samples: i2c_fujitsu: change hard-coded device name

Change to use the device config name defined by driver's Kconfig
for device binding, instead of hard-coding it which is not
portable.

Jira: ZEP-1764

Change-Id: I61ed7cfd97e20faad8f1e98dacef9384e8fefc73
Signed-off-by: Kuo-Lang Tseng <kuo-lang.tseng@intel.com>