ide-tape: remove unreachable code chunk

tape->speed_control is set to 1 in idetape_setup(), but, in calculate_speeds()
its value is tested for being 0, 1, or 2. Remove the if-branches where
tape->speed_control != 1 since they are never executed. Also, rename
calculate_speeds() by adding driver's prefix as is with the other function
names.

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