drivers: counter: Fix possible null pointer dereference

The function counter_rz_gtm_set_alarm was accessing alarm_cfg->flags and
alarm_cfg->ticks before verifying that alarm_cfg is non-NULL.

This could lead to undefined behavior or crashes if a NULL pointer is
passed.

The pointer check has been moved before any dereference to fix this bug.

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