mei: move hbuf_depth from the mei device to the hw modules

The host buffer depth is hardware specific so it's better to
handle it inside the me and txe hw modules. In me the depth
is read from register in txe it's a constant number.
The value is now retrieved via mei_hbuf_depth accessor,
while it replaces mei_hbuf_max_len.

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