USB: g_mass_storage: min(...) warning fixed

This patch fixes warning caused by calling min() macro
with arguments of different types:

  drivers/usb/gadget/f_mass_storage.c:623: warning: \
  comparison of distinct pointer types lacks a cast

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: Marek Szyprowski <m.nazarewicz@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>