HID: intel-ish-hid: add printf attribute to print_log()

Structure ishtp_device contains a logging function, print_log(), which
formats some of its parameters using vsnprintf(). Add a __printf
attribute to this function field (and to ish_event_tracer()) in order to
detect at compile time issues related to the printf-like formatting.

While at it, make format parameter a const pointer as print_log() is not
supposed to modify it.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>