ALSA: virmidi: Fix wrong error check

While rewriting the code in the previous commit [316638a5: ALSA:
virmidi: fixed code style issues], the error check was wrongly
converted.  This resulted an Oops.

Fixes: 316638a5 ('ALSA: virmidi: fixed code style issues')
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>