usb: gadget: tegra-xudc: Fix crash in vbus_draw

usb: gadget: tegra-xudc: Fix crash in vbus_draw

[ Upstream commit 5629d31955297ca47b9283c64fff70f2f34aa528 ]

Commit 2ecb9be801e0 ("usb: gadget: tegra-xudc: Add vbus_draw support")
populated the vbus_draw callback for the Tegra XUDC driver. The function
tegra_xudc_gadget_vbus_draw(), that was added by this commit, assumes
that the pointer 'curr_usbphy' has been initialised, which is not always
the case because this is only initialised when the USB role is updated.
Fix this crash, by checking that the 'curr_usbphy' is valid before
dereferencing.

Fixes: 2ecb9be801e0 ("usb: gadget: tegra-xudc: Add vbus_draw support")
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://lore.kernel.org/r/20230405181854.42355-1-jonathanh@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>