greybus: firmware: add comment to show size calculations

The firmware file name's maximum length is set to 56 right now, but it
isn't very much readable how we got to that value (unless someone reads
that line).

Update the comment to show calculations.
Reported-by: Alex Elder <alex.elder@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/firmware.h

@@ -14,7 +14,10 @@
 
 #define FW_NAME_PREFIX	"gmp_"
 
-/* Length of the string in format: "FW_NAME_PREFIX""%08x_%08x_%08x_%08x_%s.tftf" */
+/*
+ * Length of the string in format: "FW_NAME_PREFIX""%08x_%08x_%08x_%08x_%s.tftf"
+ *                                  (3 + 1 + 4 * (8 + 1) + 10 + 1 + 4 + 1)
+ */
 #define FW_NAME_SIZE		56
 
 /* Firmware Management Protocol specific functions */