eeepc-laptop: make fan1_input really read-only

In the instantiation of the fan1_input device attribute, NULL is passed
as set function to store_sys_hwmon. The function pointer is never
checked before dereferencing it. This is fine if we can guarantee that
it will never be called with an invalid pointer, but we can't. If
someone from user space decides to change the permissions on this
attribute and write to it, kernel will crash.

Introduce EEEPC_CREATE_SENSOR_ATTR_RO() to instantiate a read-only
attribute, and declare fan1_input with it. This ensures store_sys_hwmon
is never called with NULL parameters. If someone tries to write the
attribute, the system will at least keep its sanity.

This also causes EEEPC_CREATE_SENSOR_ATTR() to be only used for R/W
attributes.This enables us to drop the _mode argument from the macro
and use DEVICE_ATTR_RW() internally while we're at it. Append _RW to the
name for readability.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>