rndis_wlan: preallocate command buffer for set/get_oid

Reduce amount of kmalloc/kfree calls in set/get_oid by preallocating command
buffer.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>