staging: vt6655: fix overly large stack usage

We get a warning for the large stack usage in some configurations:

drivers/staging/vt6655/device_main.c: In function 'device_ioctl':
drivers/staging/vt6655/device_main.c:2974:1: warning: the frame size of 1304 bytes is larger than 1024 bytes [-Wframe-larger-than=]

This is addressed in linux-3.19 with commit 67013f2c ("staging: vt6655:
mac80211 conversion add main mac80211 functions"), which obsoletes the
device_ioctl() function, but as that does not apply to stable kernels,
this picks an easier way out by using dynamic allocation.

The driver was merged in 2.6.31, and the fix applies to all versions
before 3.19.

Fixes: 5449c685 ("Staging: Add pristine upstream vt6655 driver sources")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>