ath: dfs_pattern_detector: Avoid open coded arithmetic in memory allocation

kmalloc_array()/kcalloc() should be used to avoid potential overflow when
a multiplication is needed to compute the size of the requested memory.

kmalloc_array() can be used here instead of kcalloc() because the array is
fully initialized in the next 'for' loop.

Finally, 'cd->detectors' is defined as 'struct pri_detector **detectors;'.
So 'cd->detectors' and '*cd->detectors' are both some pointer.
So use a more logical 'sizeof(*cd->detectors)'.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/0fbcd32a0384ac1f87c5a3549e505e4becc60226.1640624216.git.christophe.jaillet@wanadoo.fr