isight_firmware: Avoid crash on loading invalid firmware

Different tools generate slightly different formats of the isight
firmware. Ensure that the firmware buffer is not overrun, while still
ensuring that the correct amount of data is written if trailing data is
present.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Report-by: Justin Mattock <justinmattock@gmail.com>
Tested-by: Justin Mattock <justinmattock@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>