dell-laptop: Use buffer with 32-bit physical address

Calls to communicate with system firmware via a SMI (using dcdbas)
need to use a buffer that has a physical address of 4GB or less.
Currently the dell-laptop driver does not guarantee this, and when the
buffer address is higher than 4GB, the address is truncated to 32 bits
and the SMI handler writes to the wrong memory address.

Signed-off-by: Stuart Hayes <stuart_hayes@dell.com>
Acked-by: Matthew Garrett <mjg@redhat.com>