Set proper ATA UDMA mode for bf548 according to system clock.

UDMA Mode - Frequency compatibility

UDMA5 - 100 MB/s   - SCLK  = 133 MHz
UDMA4 - 66 MB/s    - SCLK >=  80 MHz
UDMA3 - 44.4 MB/s  - SCLK >=  50 MHz
UDMA2 - 33 MB/s    - SCLK >=  40 MHz

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>