media: usb: s2255: Refactor s2255_get_fx2fw

Resize the buffer to the actual size needed and initialize it. With this
we can convince gcc-11 that the variable is not used uninitialized.

drivers/media/usb/s2255/s2255drv.c:1914:25: warning: 'transBuffer' may be used uninitialized [-Wmaybe-uninitialized]

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>