ALSA: mips/sgio2audio: Remove redundant platform_set_drvdata()

Commit 0998d063

 (device-core: Ensure drvdata = NULL when no
driver is bound) removes the need to set driver data field to
NULL.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Vivien Chappelier <vivien.chappelier@linux-mips.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>