powerpc: Fix the MODALIAS generation in modpost for of devices

Since the devices may have multiple (or none) compatible properties,
the uevent generated internally by the kernel may have multiple
"C..." entries. So the MODALIAS stored in the module must have
wilcard before and after the compatible entry.
Also, if the 'compatible' field is not used for matching, there
will be no 'C' and that must handled as well.

The previous code handled all those case incorrectly and it
"mostly" worked ... but not always.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>