staging: r8188eu: Use memdup_user instead of kmalloc/copy_from_user

Fix following coccicheck warning:
./drivers/staging/r8188eu/os_dep/ioctl_linux.c:1986:8-15: WARNING
opportunity for memdup_user.

Use memdup_user rather than duplicating its implementation, which
makes code simple and easy to understand.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20211021122015.6974-1-wanjiabing@vivo.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>