drivers: sensor: bma4xx: Avoid potential overflow

Coverity reports a potential integer overflow in the accel_range
computation due to the use of a left shift on an int type.

CID 520269: Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)

Even though the register value is constrained to 0–3 by the BMA456 spec,
and no real overflow occurs, an explicit cast to int64_t prevents false
positives and aligns with safe coding practices.

Fixes: #90517

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>