[PATCH] Update Prism54 MAINTAINERS entry

prism54-private@prism54.org bounces with

    SMTP error from remote mailer after RCPT TO:<prism54-private@prism54.org>:
    host mx1.tuxfamily.net [212.85.158.8]: 550 unknown user

developers@islsm.org seems to be the new mailing list.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>