wifi: ath6kl: minor fix for allocation size

Although the "param" pointer occupies more or equal space compared
to "*param", the allocation size should use the size of variable
itself.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: bdcd8170 ("Add ath6kl cleaned up driver")
Signed-off-by: Alexey V. Vissarionov <gremlin@altlinux.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230117110414.GC12547@altlinux.org