staging: rtl8192u: use kzalloc to allocate firmware pointer

the firmware pointer is allocated with kmalloc and memset,
instead we can just do a kzalloc which will return a memory that
has been memset.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>