crypto/qat: fix Arm build with special memcpy

crypto/qat: fix Arm build with special memcpy

In Arm platform, when "RTE_ARCH_ARM64_MEMCPY" is set as true, gcc will
report variable uninitialized warning:

../drivers/crypto/qat/qat_sym_session.c: In function ‘partial_hash_compute’:
../lib/eal/include/generic/rte_byteorder.h:241:24: warning:
‘<U35a0>’ may be used uninitialized in this function
	[-Wmaybe-uninitialized]
	241 | #define rte_bswap32(x) __builtin_bswap32(x)
	...

This is because "digest" will be initialized by "rte_memcpy" function
rather than "memcpy" if "RTE_ARCH_ARM64_MEMCPY" is set as true.
Furthermore, 'rte_memcpy' will initialize 'digest' with two steps
by invoking rte_mov_x functions. For example:

partial_hash_sha1 -> rte_memcpy -> rte_memcpy_ge16_lt_128 ->
step 1: rte_mov16(dst,src )
step 2: rte_mov16(dst - 16 + n, src - 16 + n)

However, gcc compiler cannot identify this multi-step initialization,
then it will report warning.

To fix this, use "memset" to initialize "digest".

Fixes: cd7fc8a84b48 ("eal/arm64: optimize memcpy")
Cc: stable@dpdk.org
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Adam Dybkowski <adamx.dybkowski@intel.com>