USB: musb: fix compiling warning with min() macro

Current musb gadget dma code produces the warning:
drivers/usb/musb/musb_gadget.c: In function 'txstate':
drivers/usb/musb/musb_gadget.c:312: warning: comparison of distinct
                                             pointer types lacks a cast

So switch to min_t(size_t, ...).

Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>