Revert "tipc: check the bearer min mtu properly when setting it by netlink"

This reverts commit 673cb479 which is
commit 35a089b5 upstream.

It breaks the Android kernel abi, so revert it for now.  If it is needed
in the future, it can be brought back in an ABI-safe way.

Bug: 161946584
Change-Id: I97b5bb9a4c40c5cdf1f20b612782917e166fbe8c
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>