e1000: Fixing interrupts pace.

e1000: Fixing interrupts pace.

This patch introduces an upper bound for number of interrupts
per second. Without this bound an interrupt storm can occur as
it has been observed on Windows 10 when disabling the device.

According to the SPEC - Intel PCI/PCI-X Family of Gigabit
Ethernet Controllers Software Developer's Manual, section
13.4.18 - the Ethernet controller guarantees a maximum
observable interrupt rate of 7813 interrupts/sec. If there is
no upper bound this could lead to an interrupt storm by e1000
(when mit_delay < 500) causing interrupts to fire at a very high
pace.
Thus if mit_delay < 500 then the delay should be set to the
minimum delay possible which is 500. This can be calculated
easily as follows:

Interval = 10^9 / (7813 * 256) = 500.

Signed-off-by: Sameeh Jubran <sameeh@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>