mei: iamthif: use client write functions

Reduce code duplication in amthif code by reusing
regular client write functions.
Add completed flag to cb so amthif client can add
rx credits on write completion

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