drivers: sensor: bmp581: add const qualifier when casting

Add the 'const' qualifier when casting 'dev->config' to 'struct
bmp581_config *' to preserve const-correctness and fix SonarQube warning :

 "A cast shall not remove any const or volatile qualification from the
 type of a pointer or reference."

This improves maintainability by respecting the const contract and resolve
static analysis issues.

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