ALSA: sound/isa/gus: Correct code taking the size of a pointer

sizeof(share_id) is just the size of the pointer.  On the other hand,
block->share_id is an array, so its size seems more appropriate.

A simplified version of the semantic patch that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression *x;
expression f;
type T;
@@

*f(...,(T)x,...)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>