ARM: tegra: clear $usb_need_init each boot

ARM: tegra: clear $usb_need_init each boot

$usb_need_init prevents "usb start" from being run multiple times for
each boot attempt, i.e. once for USB storage, another for PXE, and
another for DHCP. However, the flag that's used to determine when to run
"usb start" is never cleared, so a subsequent "boot" command will never
probe for a freshly plugged in USB device. Fix this so that new USB
devices will be probed once per boot attempt.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>