staging: vme_user: refactor llseek to switch(){}

This makes vme_user_llseek ignore all minors that don't have llseek
implementation.

Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
Cc: Igor Alekseev <igor.alekseev@itep.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>