e1000e: Simplify MSI interrupt testing

The code is quite convoluted, simplify it. This also avoids calling
e1000_request_irq() without testing the value it returned, which was
bad.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Bruce Allan <bruce.w.allan@intel.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>