Fixes gcc '-Wunused-but-set-variable' warning:

drivers/crypto/xilinx/zynqmp-aes-gcm.c: In function 'zynqmp_aes_aead_cipher':
drivers/crypto/xilinx/zynqmp-aes-gcm.c:83:30: warning:
 variable 'drv_ctx' set but not used [-Wunused-but-set-variable]

commit bc86f9c5 ("firmware: xilinx: Remove eemi ops for aes engine") left
behind this, remove it.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20200505101200.195184-1-yuehaibing@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>