mpsse: Defer errors until flush

mpsse: Defer errors until flush

Simplify the API by making all MPSSE command functions return void instead
of an error code. If there is an error during an implicit flush in a
command call, further commands are ignored until an explicit flush is
performed. The flush function returns and clears any error code set.

The only command functions that still return an error code are those that
can fail directly based on the type of the FTDI chip, i.e. when trying to
enable RCLK or divide-by-5 on a non-high-speed chip.

Adapt the ftdi adapter driver to the new API.

Change-Id: I12979c723c81f7fd022c25821b029112f02b3f95
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1499
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>