serial_core: fix sizeof(pointer)

sizeof when applied to a pointer typed expression gives the
size of the pointer.

Generated by: scripts/coccinelle/misc/noderef.cocci

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>