staging: mt7621-mmc: add missing header.

<linux/interrupt.h> is sometimes included by other include files,
and sometimes not, depending on config, particularly CONFIG_HIGHMEM.

So include it explicitly rather than relying on implicit inclusion.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>