[rng] Check for several functioning RTC interrupts

Commit 74222cd ("[rng] Check for functioning RTC interrupt") added a
check that the RTC is capable of generating interrupts via the legacy
PIC, since this mechanism appears to be broken in some Hyper-V virtual
machines.

Experimentation shows that the RTC is sometimes capable of generating
a single interrupt, but will then generate no subsequent interrupts.
This currently causes rtc_entropy_check() to falsely detect that the
entropy gathering mechanism is functional.

Fix by checking for several RTC interrupts before declaring that it is
a functional entropy source.

Reported-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

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