wifi: wfx: Use devm_kmemdup to replace devm_kmalloc + memcpy

Use the helper function devm_kmemdup() rather than duplicating its
implementation, which helps to enhance code readability.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230810114939.2104013-1-lizetao1@huawei.com