drivers: sensor: fxls8974: fix size misalignment memory access

Coverity identified out-of-bounds access. A uint8_t being cast to
an enum is undefined in that the enum can be allocated 4bytes.

Change the internal function to use the base type of the variable,
uint8_t to avoid potential compiler size alignement problems.

Fixes #81927

Signed-off-by: David Leach <david.leach@nxp.com>