lightrec: Fix unmap size of BIOS

The BIOS was mapped as 2 MiB, since we want to use a huge page if
possible. Therefore it should also be unmapped as 2 MiB, otherwise the
upper 1.5 MiB will still be mapped after de-init, which will make it
impossible to map the BIOS once again.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>