net: netcp: rework the code for get/set sw_data in dma desc

SW data field in descriptor can be used by software to hold private
data for the driver. As there are 4 words available for this purpose,
use separate macros to place it or retrieve the same to/from
descriptors. Also do type cast of data types accordingly.

Cc: Wingman Kwok <w-kwok2@ti.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
CC: Arnd Bergmann <arnd@arndb.de>
CC: Grygorii Strashko <grygorii.strashko@ti.com>
CC: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>