samples: sensor: Fix logically dead code in bmg160 sample

Converts the bmg160 sample application to check the sensor device at
build time instead of runtime. This fixes a Coverity issue for logically
dead code introduced in commit 5832e588e3664cc0abb76847ac3a5e4d1954392d.

Fixes #35118
Coverity-CID: 235919

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>