mptscsih_slave_destroy issues an unconditional synchronize cache.
It is not necessary for fibre channel devices for which the rport
has been deleted.  This patch creates a new function,
mptfc_slave_destroy(), which implements the appropriate test.

Additionally, Eric's review of the original version of this patch
pointed me at some unnecessary tests of the rport dd_data field.
I've removed one and made two others bugs.

Signed-off-by: Michael Reed <mdr@sgi.com>