usb: gadget: atmel: reliably generate disconnect by disabling controller

usb: gadget: atmel: reliably generate disconnect by disabling controller

Contrary to the datasheet, setting both DETACH and PULLD_DIS bits to 1
does not always drive the DP and DM lines to high-impedance. This
prevents the host from reliably detecting a USB disconnect and subsequent
reconnect.

The symptom is that the first gadget command (e.g., dhcp) succeeds, while
subsequent commands (e.g., nfs) fail.

Disabling and re-enabling the controller entirely, instead of toggling the
PULLD_DIS bit, reliably generates a disconnect event.

The Linux driver works correctly because gadget_disconnect/gadget_connect
are always followed by gadget_udc_start/gadget_udc_stop. In U-Boot
pullup() is used solely.

This behavior has been observed on the SAM9X60-Curiosity and
AT91SAM9G25-EK boards and has been reported to Microchip.

Signed-off-by: Zixun LI <admin@hifiphile.com>
Link: https://lore.kernel.org/r/20250602-pullup-v1-1-edcde5a050dd@hifiphile.com
[mkorpershoek: reworded commit title + comment to usba_udc_pullup()]
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>