[media] rtl2832_sdr: do not use dynamic stack allocation

Do not use dynamic stack allocation.

>> drivers/staging/media/rtl2832u_sdr/rtl2832_sdr.c:181:1:
warning: 'rtl2832_sdr_wr' uses dynamic stack allocation [enabled by default]

Reported-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>