tpm: Do not dereference NULL pointer if acpi_os_map_memory() fails.

In drivers/char/tpm/tpm_acpi.c::read_log() we call
acpi_os_map_memory(). That call may fail for a number of reasons
(invalid address, out of memory etc). If the call fails it returns
NULL and we just pass that to memcpy() unconditionally, which will go
bad when it tries to dereference the pointer.

Unfortunately we just get NULL back, so we can't really tell the user
exactely what went wrong, but we can at least avoid crashing and
return an error (-EIO seemed more generic and more suitable here than
-ENOMEM or something else, so I picked that).

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Kent Yoder <key@linux.vnet.ibm.com>