drivers: sensor: ens210: Fix broken CONFIG_ENS210_CRC_CHECK checks

The CONFIG_* prefix was missing, making the #ifdefs always false.

Found with a script (CONFIG_ENS210_CRC_CHECK was unused).

Also make ens210_crc7() static. Guessing it's unused outside this file.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>