usb: ehci-omap: Don't softreset USB High-speed Host (UHH) Module

Fixes NFS root problems with Beagle (3530 ES1.0) when used with
external USB-ethernet adapter and "USB start" command used within
u-boot.

Soft resetting the UHH module causes instability issues on
all OMAPs so we just avoid it.

See OMAP36xx Errata
  i571: USB host EHCI may stall when entering smart-standby mode
  i660: USBHOST Configured In Smart-Idle Can Lead To a Deadlock

On OMAP4/5, soft-resetting the UHH module can put it into
Smart-Idle mode and lead to a deadlock.

On OMAP3 this doesn't seem to be the case but still instabilities
are observed on beagle (3530 ES1.0) if soft-reset is used.
 e.g. NFS root failures with Linux kernel.

Signed-off-by: Roger Quadros <rogerq@ti.com>