crypto: inside-secure - initialize first_rdesc to make GCC happy

In the cipher safexcel_send_req function, GCC warns that
first_rdesc may be used uninitialized. While this should never
happen, this patch removes the warning by initializing this
variable to NULL to make GCC happy.

This was reported by the kbuild test robot.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>