ide: insert BUG_ON() into __ide_set_handler() (take 2)

Replace the check for hwgroup->handler and printk(KERN_CRIT, ...) at the start
of __ide_set_handler() with mere BUG_ON() while removing such from the caller,
ide_execute_command(). Fix up the code formatting, while at it...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>