mwifiex: replace kmalloc & memcpy sequence with kmemdup

Sequence of kmalloc/kzalloc and memcpy is replaced with
kmemdup.

Cc: Walter Harms <wharms@bfs.de>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>