lg2: Reset format string for multiple uses
lg2: Reset format string for multiple uses

The stderr output format string pointer was incremented during
processing, and never reset, resulting in empty output to cerr on all
log messages beyond the first. This resets the pointer on each call.

Tested: Issued multiple lg2::info calls and observed them all on stderr.

Signed-off-by: Jonathan Doman <81634b64c36b51cdf9cf4576937c86cbf71125f0@intel.com>
Change-Id: I1a2b3eaa23604b5260ee61c3167f6956018f034d

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging