Remove redundant dev->si_drv1 NULL checks in the LinuxKPI.
This pointer is checked during the linux_dev_open() callback and does
not need to be NULL checked again. It should always be set for
character devices belonging to the "linuxcdevsw" and technically
there is no need to NULL check this pointer at all.

Suggested by: kib @
MFC after: 1 week
Sponsored by: Mellanox Technologies