ide: move common code out of tf_load() method

Move device register masking (and setting drive->select) out of tf_load() method
and into the only function that needs to use this code, do_rw_taskfile()...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
[bart: fix whitespace error]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>