net/igb: fix memcpy length

net/igb: fix memcpy length

The size of "flex_filter.filter_info.mask" and "filter->mask" are 16
bytes, but the length of memcpy--"RTE_ALIGN(filter->len, sizeof(char))
/ sizeof(char)" may reach 128 bytes which may cause array access out
of bound.

Fix it by replacing "sizeof(char)" by "CHAR_BIT".

Fixes: 231d43909a31 ("igb: migrate flex filter to new API")
Cc: stable@dpdk.org
Signed-off-by: Yong Wang <wang.yong19@zte.com.cn>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>