ide: Fix an overrun found in the CS5535 IDE driver

As found by the Coverity checker, and reported by Adrian Bunk, this
fixes a overrun error in the CS5535 IDE driver.  Somebody got a little
excited with the if() statement - the CS5535 only supports UDMA 0-4.

Bart:
Not a bug per se since the upper layer will never feed this function
with speed > XFER_UDMA_4 (thanks to ->ultra_mask being set to 0x1f).

Worth fixing anyway.

Signed-off-by: Jordan Crouse <jordan.crouse@amd.com>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>