staging: rtl8188eu: os_dep: Replaced kzalloc and memcpy with kmemdup

Replaced calls to kzalloc followed by memcpy with a single call to
kmemdup.

Patch found using coccicheck.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>