rndis_wlan: fix broken data copy

Replace broken code that attempted to copy 6 byte array to 64-bit
integer. Due to missing cast to 64-bit integer, left shift operation
were 32-bit and lead to bytes been copied over each other. New code
uses simple memcpy, for greater readability and efficiency.

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