[media] em28xx-i2c: rt_mutex_trylock() returns zero on failure

The code is checking for negative returns but it should be checking for
zero.

Fixes: aab3125c

 ('[media] em28xx: add support for registering multiple i2c buses')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>