INSTALL.Debian: Update instructions related to package ordering.

If we install the userspace packages first, without a kernel module
present in the correct path, we get an error because package install
automatically tries to start the userspace daemons and we look for
a kernel module to load.

Installing a kernel module package first and then following it by
userspace package sounds like the correct way to do it. This change
in ordering helps my next commit.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>