igb: Rename a variable in igb_receive_internal()

Rename variable "n" to "causes", which properly represents the content
of the variable.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Signed-off-by: Jason Wang <jasowang@redhat.com>