[media] hackrf: Fix a long constant

drivers/media/usb/hackrf/hackrf.c:64:3: warning: this decimal constant is unsigned only in ISO C90 [enabled by default]
   .rangehigh  = 4294967294, /* max u32, hw goes over 7GHz */
   ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>