USB: Remove unnecessary casts of void ptr returning alloc function return values

Hi,

The [vk][cmz]alloc(_node) family of functions return void pointers which
it's completely unnecessary/pointless to cast to other pointer types since
that happens implicitly.

This patch removes such casts from drivers/usb/

Signed-off-by: Jesper Juhl <jj@chaosbits.net>