mv88e6xxx: Fix uninitialized variable warning.

In mv88e6xxx_probe(), ("np" or "pdata") might be an invariant
but GCC can't see that, therefore:

drivers/net/dsa/mv88e6xxx/chip.c: In function ‘mv88e6xxx_probe’:
drivers/net/dsa/mv88e6xxx/chip.c:4420:13: warning: ‘compat_info’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  chip->info = compat_info;

Actually, it should have warned on the "if (!compat_info)" test, but
whatever.

Explicitly initialize to NULL in the variable declaration to
deal with this.

Fixes: 877b7cb0

 ("net: dsa: mv88e6xxx: Add minimal platform_data support")
Signed-off-by: David S. Miller <davem@davemloft.net>