V4L/DVB (6350): V4L: possible leak in em28xx_init_isoc

Coverity (CID 1929) spotted the following: if a transfer buffer
allocation fails, the last allocated urb is leaked (it hasn't been
stored in dev->urb[] yet so em28xx_uninit_isoc misses it). The patch
also includes a small typo fix.

Signed-off-by: Florin Malita <fmalita@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>