misc: tifm: Remove VLA

There's an ongoing effort to remove VLAs[1] from the kernel to eventually
turn on -Wvla. The single VLA can either take a value of 2 or 4 so switch
to the upper bound.

[1] https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Laura Abbott <labbott@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>