V4L/DVB (6308): V4L: zc0301, remove bad usage of ERESTARTSYS

zc0301, remove bad usage of ERESTARTSYS

down_read_trylock can't be interrupted and so ERESTARTSYS would reach
userspace, which is not permitted. Change it to EAGAIN

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Luca Risolia <luca.risolia@studio.unibo.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>