eeprom: fix memory leak on buf when failed allocation of csraddr_str

The error return path When csraddr_str fails to free buf, causing a
memory leak. Fix this by returning via the free_buf label that
performs the necessary cleanup.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>