When ivtv PCM device is accessed at the state where no firmware is
loaded, it oopses like:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
  IP: [<ffffffffa049a881>] try_mailbox.isra.0+0x11/0x50 [ivtv]
  Call Trace:
    [<ffffffffa049aa20>] ivtv_api_call+0x160/0x6b0 [ivtv]
    [<ffffffffa049af86>] ivtv_api+0x16/0x40 [ivtv]
    [<ffffffffa049b10c>] ivtv_vapi+0xac/0xc0 [ivtv]
    [<ffffffffa049d40d>] ivtv_start_v4l2_encode_stream+0x19d/0x630 [ivtv]
    [<ffffffffa0530653>] snd_ivtv_pcm_capture_open+0x173/0x1c0 [ivtv_alsa]
    [<ffffffffa04526f1>] snd_pcm_open_substream+0x51/0x100 [snd_pcm]
    [<ffffffffa0452853>] snd_pcm_open+0xb3/0x260 [snd_pcm]
    [<ffffffffa0452a37>] snd_pcm_capture_open+0x37/0x50 [snd_pcm]
    [<ffffffffa033f557>] snd_open+0xa7/0x1e0 [snd]
    [<ffffffff8118a628>] chrdev_open+0x88/0x1d0
    [<ffffffff811840be>] do_dentry_open+0x1de/0x270
    [<ffffffff81193a73>] do_last+0x1c3/0xec0
    [<ffffffff81194826>] path_openat+0xb6/0x670
    [<ffffffff81195b65>] do_filp_open+0x35/0x80
    [<ffffffff81185449>] do_sys_open+0x129/0x210
    [<ffffffff815b782d>] system_call_fastpath+0x1a/0x1f

This patch adds the check of firmware at PCM open callback like other
open callbacks of this driver.

Bugzilla: https://apibugzilla.novell.com/show_bug.cgi?id=875440

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>