staging: rtl8723au: Use kmemdup() instead of memcpy() to duplicate memory

This issue was reported by coccicheck using the semantic patch
at scripts/coccinelle/api/memdup.cocci, and tested by compilation.

Signed-off-by: Benoit Taine <benoit.taine@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>