staging: nvec: Avoid the use of BUG_ON

Prevent a kernel panic by avoiding the use of the BUG_ON macro.
Checkpatch detected this issue.

The BUG_ON macro is not needed as such cases shouldn't happen and they
were introduced for debugging purposes.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Acked-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>