Fix the issue of dialing 8 or more digits

struct ast_channel *c from sub->parent whose type is brcm_pvt not acting as expected, the ast_channel_exten_set(c, p->ext) seems handled with some wrong memory address that leads to the issue.

Using the ast_copy_string instead to set the sub->parent->ext and sub->parent->context directly, which will be used by brcm_new(line 1918).