dmaengine: jz4740: Fix GPL v2 license string typo

Per license_is_gpl_compatible(), the MODULE_LICENSE() string for GPL v2 is
"GPL v2", not "GPLv2".  Use "GPL v2" so this module doesn't taint the
kernel.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>