mei: move hw dependent functions to interface.c

1. move direct register handling to interface.c and make them static
2. add new function mei_clear_interrupts that wraps direct register
access
3. export other functions in mei_dev.h

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