programmer_delay: filter 0 usec delays

We use 0 as delay value for some chips. Just skipping these here is the
most elegant, maintainable solution.

Corresponding to flashrom svn r1760.
Signed-off-by: Urja Rannikko <urjaman@gmail.com>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>

flashrom.c

@@ -487,7 +487,8 @@ void chip_readn(const struct flashctx *flash, uint8_t *buf, chipaddr addr,
 
 void programmer_delay(int usecs)
 {
-	programmer_table[programmer].delay(usecs);
+	if (usecs > 0)
+		programmer_table[programmer].delay(usecs);
 }
 
 void map_flash_registers(struct flashctx *flash)