ide: fix registers loading order for IDE_NSECTOR_REG in execute_drive_cmd()

Move loading of IDE_NSECTOR_REG from ide_cmd() to execute_drive_cmd()
(load the IDE_NSECTOR_REG just after IDE_FEATURE_REG).

This also allows us to drop 'nsect' argument from ide_cmd() and simplify
execute_drive_cmd() code for REQ_TYPE_ATA_CMD case a bit.

It shouldn't affect anything (just a usual paranoia to separate changes
which change the way in which hardware is accessed from code cleanups).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>