mei: don't query HCSR for host buffer depth

1. We record host (write) buffer depth during reset
 so we don't need to query HCSR register later on.
 The host buffer depth doesn't change after the reset
2. Use mei_hbuf_max_data function to compute payload size in bytes

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