usb: gadget: atmel_usba: fix crash when no endpoint are specified

usb: gadget: atmel_usba: fix crash when no endpoint are specified

If no endpoints are present in the device tree, the kernel will crash with the
following error:

Unable to handle kernel paging request at virtual address 00101008
[...]
[<c0222ff4>] (composite_dev_prepare) from [<c022326c>] (composite_bind+0x5c/0x190)
[<c022326c>] (composite_bind) from [<c021ff8c>] (udc_bind_to_driver+0x48/0xf0)
[<c021ff8c>] (udc_bind_to_driver) from [<c02208e0>] (usb_gadget_probe_driver+0x7c/0xa0)
[<c02208e0>] (usb_gadget_probe_driver) from [<c0008970>] (do_one_initcall+0x94/0x140)
[<c0008970>] (do_one_initcall) from [<c04b4b50>] (kernel_init_freeable+0xec/0x1b4)
[<c04b4b50>] (kernel_init_freeable) from [<c0376cc4>] (kernel_init+0x8/0xe4)
[<c0376cc4>] (kernel_init) from [<c0009590>] (ret_from_fork+0x14/0x24)
Code: e5950014 e1a04001 e5902008 e3a010d0 (e5922008)
---[ end trace 35c74bdd89b373d0 ]---
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

This checks for that case and returns an error, not allowing the driver to be
loaded with no endpoints.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>