stlink: simplify handling of SWIM

stlink: simplify handling of SWIM

Now that SWIM is not accessed through HLA anymore, decouple the
SWIM code and remove the conditional execution.

Fix the inconsistency of the return type for stlink_usb_state() in
case of SWIM (returns ERROR_OK while type is enum target_state)
introduced by commit 3de6b5f6e52f ("jtag/drivers/stlink_usb :
implemented and repaired SWIM support").
The code added by commit above in stlink_usb_state() is an hack to
reuse existing HLA API to perform a reconnect.
Move the SWIM specific code from stlink_usb_state() to a dedicated
stlink_swim_op_reconnect() that provides consistent data type.

Change-Id: I3fe175fef00b0735bea6139b057f217a080c9d38
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 3de6b5f6e52f ("jtag/drivers/stlink_usb : implemented and repaired SWIM support")
Reviewed-on: http://openocd.zylin.com/5532
Tested-by: jenkins