Fix comment that limits num_bits to 32.

Andreas Fritiofson says "If any adapter driver does not work with
arbitrary lengths of individual fields, it's a bug."
https://sourceforge.net/p/openocd/mailman/message/35091945/

Note also that lengths of at least 96 bits are already in use, eg. in
mips_ejtag_add_scan_96().

Change-Id: I62a150adc75c0ef78827683ca8d0a8e90310a982
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/3491
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

src/jtag/jtag.h

@@ -107,7 +107,7 @@ extern tap_state_t cmd_queue_cur_state;
  * jtag_add_dr_scan_check() to validate the value that was scanned out.
  */
 struct scan_field {
-	/** The number of bits this field specifies (up to 32) */
+	/** The number of bits this field specifies */
 	int num_bits;
 	/** A pointer to value to be scanned into the device */
 	const uint8_t *out_value;