wireless: at76c50x: allocating too much data

This is a cut and paste mistake, sizeof(struct mib_local) was intended
instead of sizeof(struct mib_phy).  The call to at76_get_mib() uses
sizeof(struct mib_local) correctly, although I changed that to
sizeof(*m) for style reasons after discussion with some of the wireless
maintainers.

The current code works fine because mib_phy structs are larger than
mib_local structs.  But we may as well clean it up.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>