Fix platform_get_snd_device_name_extn functions.

This was showing up as a const cast, but those lines were completely
wrong. C does not work that way. Replace them with a strlcpy which
appears to be what was intended.

Note this removes a device_name NULL check. Looking through callers, no
one ever passes in NULL, and no other platform_get_snd_device_name_extn
implementation ever checks for it.

Test: Still compiles. Device boots.
Change-Id: I5d71550f8f3891aacdf8d8aa95c3cd6094aa4701