x86/platform/intel-mid: Get rid of duplication of IPC handler

x86/platform/intel-mid: Get rid of duplication of IPC handler

There is no other device handler than ipc_device_handler() and sfi.c already
has a handler for IPC devices.

Replace a pointer to custom handler by a flag. Due to this change adjust
sfi_handle_ipc_dev() to handle it instead of ipc_device_handler().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170105130235.177792-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>