[media] media: stk1160: Avoid stack-allocated buffer for control URBs

Currently stk1160_read_reg() uses a stack-allocated char to get the
read control value. This is wrong because usb_control_msg() requires
a kmalloc-ed buffer.

This commit fixes such issue by kmalloc'ating a 1-byte buffer to receive
the read value.

While here, let's remove the urb_buf array which was meant for a similar
purpose, but never really used.

Cc: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: stable@vger.kernel.org      # for v3.7 and up
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>