improve modalias building

For one, there's no point in the respective pieces to be rebuilt
unconditionally on each and every rebuild.

Second there's no need to invent a custom rule for generating the .s
file from the .c source - we can simply use the generic rule here.

And finally, $(obj) should be used to refer to files in the build tree
(rather than spelling out the subdirectory).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>