staging:line6: Fix use of variable length arrays

This patch fixes the sparse warning:
drivers/staging/line6/midi.c:50:34: warning: Variable length array is used.
The size is same as the variable LINE6_FALLBACK_MAXPACKETSIZE,
so use that.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>