dev_name is supposed to return the name of the device plus the id

currently we use %s%d format where in the kernel the use %s.%d
we may think to switch to this format for the device name and keeping the %s%d
for the devfs

this will be usefull to not modify the clock device name as example

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>