fsi: master: Add boolean parameter to link_enable function

Add the ability to disable a link with a boolean parameter to the
link_enable function. This is necessary so that the master can disable
links that it isn't using; for example, links to slaves that fail
initialization.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>