octeontx2: fix -Wnonnull warning

When compile testing this driver on a platform on which probe() is
known to fail at compile time, gcc warns about the cgx_lmactype_string[]
array being uninitialized:

In function 'strncpy',
    inlined from 'link_status_user_format' at /git/arm-soc/drivers/net/ethernet/marvell/octeontx2/af/cgx.c:838:2,
    inlined from 'cgx_link_change_handler' at /git/arm-soc/drivers/net/ethernet/marvell/octeontx2/af/cgx.c:853:2:
include/linux/fortify-string.h:27:30: error: argument 2 null where non-null expected [-Werror=nonnull]
   27 | #define __underlying_strncpy __builtin_strncpy

Address this by turning the runtime initialization into a fixed array,
which should also produce better code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>