[ZD1211RW]: Don't needlessly initialize variable to NULL in zd_chip

No need to initialize to NULL when variable is never used before
it's assigned the return value of a kmalloc() call.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>