rngd: fix --debug to also filter syslog() calls

rngd: fix --debug to also filter syslog() calls

Debug logs were only controlled by --debug flag while in --foreground mode.
In --daemon mode (the default for us) /var/log/message got stuffed with
details of entropy pool refilling, which is useless in production, and
hamful when log rotation then gets rid of the more useful logs.

This change makes the two modes consistently only produce debug logs when
--debug is specified.

Signed-off-by: Yann Dirson <yann@blade-group.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>