usb: gadget: fsl_udc_core: fixup struct_udc_setup documentation

The original implementation from commit b504882d
("USB: add Freescale high-speed USB SOC device controller driver")
returned NULL on failure and an allocated + initialized struct fsl_udc on
success. The current code introduced in commit 4365831d


("USB: fsl_usb2_udc: Get max ep number from DCCPARAMS register") only
provides partial initialization as well as returning 0 on success and -1
on failures. The function documentation is updated accordingly.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: 4365831d

 ("USB: fsl_usb2_udc: Get max ep number from DCCPARAMS register")
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>