mei: iamthif: use regular client read functions

Reduce code duplication in amthif by reusing
regular client read functions.

The change also removes the need for amthif
own buffering

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>