ARM: ADIv5, deadcode cleanup

ARM: ADIv5, deadcode cleanup

I have no idea what the scan_inout_check() was *expecting* to achieve by
issuing a read of the DP_RDBUFF register.  But in any case, that code was
clearly never being called ("invalue" always NULL) ... so remove it, and
the associated comment.

Also rename it as ap_write_check(), facilitating a cleanup of its single
call site by removing constant parameters.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>