flash: handle zero when reading stm32 flash size reg

Some variants read 0 for the flash size register, rather than
failing lets assume we have max flash fitted.

Change-Id: Ie1fb4e73606f49268a6fd5921c3aef75bc4790d3
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/744
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/flash/nor/stm32f2x.c

@@ -605,6 +605,11 @@ static int stm32x_probe(struct flash_bank *bank)
 		flash_size_in_kb = 0xffff;
 	}
 
+	/* some variants read 0 for flash size register
+	 * use a max flash size as a default */
+	if (flash_size_in_kb == 0)
+		flash_size_in_kb = 0xffff;
+
 	if ((device_id & 0xfff) == 0x411) {
 		/* check for early silicon */
 		if (flash_size_in_kb == 0xffff) {