crypto: cavium/nitrox - Remove redundant initialization of 'sg'

Pointer 'sg' is being initialized however this value is never
read as 'sg' is assigned a same value in for_each_sg().
Remove the redundant assignment.

Cleans up clang warning:

drivers/crypto/cavium/nitrox/nitrox_reqmgr.c:161:22: warning: Value
stored to 'sg' during its initialization is never read
[clang-analyzer-deadcode.DeadStores]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>