usb: gadget: f_midi: Avoid invalid pointer access

Currently f_midi function driver is marking the f_midi->card
NULL when the card is unregistered. There is a possibility that
alsa_show is called after the card is unregistered which can lead
to page fault since midi->card is assigned to rmidi->card. Avoid
this by adding midi->card pointer check in alsa_show.

Change-Id: I24fd35fb9cd35fcd5d8698b808459310b09675f9
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>