target/adi_v5_swd: improve error check while updating DP_SELECT

target/adi_v5_swd: improve error check while updating DP_SELECT

Write to register DP_SELECT can fail, but both functions
swd_queue_dp_bankselect() and swd_queue_ap_bankselect() return
void and do not propagate the error.

Change the return type of the two functions to int and check the
returned value.
Invalidate the cached value of DP_SELECT if the write fails.

Change-Id: Iba6ef8b0c2332e7f7efb66337d558fb7a4a0d39c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4980
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>