[efi] Run at TPL_CALLBACK to protect against UEFI timers

As noted in the comments, UEFI manages to combines the all of the
worst aspects of both a polling design (inefficiency and inability to
sleep until something interesting happens) and of an interrupt-driven
design (the complexity of code that could be preempted at any time,
thanks to UEFI timers).

This causes problems in particular for UEFI USB keyboards: the
keyboard driver calls UsbAsyncInterruptTransfer() to set up a periodic
timer which is used to poll the USB bus.  This poll may interrupt a
critical section within iPXE, typically resulting in list corruption
and either a hang or reboot.

Work around this problem by mirroring the BIOS design, in which we run
with interrupts disabled almost all of the time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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