net: cavium: change NET_VENDOR_CAVIUM to bool

CONFIG_NET_VENDOR_CAVIUM is only used to hide/show config options and to
include subdirectories in the build, so it doesn't make sense to make it
tristate.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>