[media] pwc: Use one shared usb command buffer

The pwc driver used to:
1. kmalloc a buffer
2. memcpy data to send over usb there
3. do the usb_control_msg call (which does not work with data on the stack)
4. free the buffer

For every usb command send. This patch changes the code to instead malloc
a buffer for this purpose once and use it everywhere.

[mchehab@redhat.com: Fix a compilation breakage with allyesconfig:
 drivers/media/video/pwc/pwc-ctrl.c: In function ‘pwc_get_cmos_sensor’:
 drivers/media/video/pwc/pwc-ctrl.c:546:3: warning: passing argument 4 of ‘recv_control_msg’ makes integer from pointer without a cast [en$
 drivers/media/video/pwc/pwc-ctrl.c:107:12: note: expected ‘int’ but argument is of type ‘unsigned char *’
 drivers/media/video/pwc/pwc-ctrl.c:546:3: error: too many arguments to function ‘recv_control_msg’
 drivers/media/video/pwc/pwc-ctrl.c:107:12: note: declared here]

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>