Fix a bunch of -Wcast-qual warnings in uhid.c

[dim]

Summary

When testing with clang 3.6.0, I got a number of the following -Wcast-qual warnings:

sys/dev/usb/input/uhid.c:737:33: error: cast from 'unsigned char const (*)[125]' to 'void *' drops const qualifier [-Werror,-Wcast-qual]
                        sc->sc_repdesc_ptr = (void *)&uhid_graphire_report_descr;
                                                     ^
sys/dev/usb/input/uhid.c:758:33: error: cast from 'unsigned char const (*)[161]' to 'void *' drops const qualifier [-Werror,-Wcast-qual]
                        sc->sc_repdesc_ptr = (void *)&uhid_graphire3_4x5_report_descr;
                                                     ^
sys/dev/usb/input/uhid.c:778:32: error: cast from 'unsigned char const (*)[166]' to 'void *' drops const qualifier [-Werror,-Wcast-qual]
                sc->sc_repdesc_ptr = (void *)&uhid_xb360gp_report_descr;
                                             ^

As far as I can see, sc_repdesc_ptr can't be made into a const pointer, so this has to be fixed using __DECONST().

Test Plan

This compiles, and the kernel boots.

[dim]

Btw, I did not wrap the lines at 80 chars, since the existing style in the file already did not do that. :)

[• hselasky]

Looks good!

[dim]

Closed by commit rS277974 (authored by @dim).