[efi] Always map full length of coherent DMA buffer allocation

[efi] Always map full length of coherent DMA buffer allocation

The EFI PCI API takes a page count as the input to AllocateBuffer()
but a byte count as the input to Map().  There is nothing in the UEFI
specification that requires us to map exactly the allocated length,
and no systems have yet been observed that will fail if the map length
does not exactly match the allocated length.  However, it is plausible
that some implementations may fail if asked to map a length that does
not match the length of the corresponding allocation.

Avoid potential future problems by always mapping the full allocated
length.

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