usb: gadget: tegra: check ep->desc before access

usb: gadget: tegra: check ep->desc before access

This changes adds condition to check ep->desc
before accessing its contents. ep->desc seems to
get Null on releasing the spin_lock.

Bug 1401730

Change-Id: I48b4c880ced0cd131bd45840839e6635fc1a219c
Signed-off-by: Rohith Seelaboyina <rseelaboyina@nvidia.com>
Reviewed-on: http://git-master/r/362366
(cherry picked from commit 4c18848a52973987a691a871c127e54d697c60dc)
Reviewed-on: http://git-master/r/362368
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Venkat Moganty <vmoganty@nvidia.com>