net: e1000e: fix dead code in e1000e_write_packet_to_guest

Because is_first is declared inside a loop, it is always true.  The store
is dead, and so is the "else" branch of "if (is_first)".  is_last is
okay though.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>