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>