netdev_tap: don't allocate DEBUG_EXTRA_STACKSIZE twice

[basilfx]

Contribution description

While browsing some code, I noticed that DEBUG_EXTRA_STACKSIZE is used at two places in the auto initialization of netdev_tap. I presume that the second time it was used incorrectly, because when creating the thread, it specified the stack size TAP_MAC_STACKSIZE.

Testing procedure

I don't have het tap device set up on macOS, but I don't think this change impacts anything, except reducing the actual allocated stack size.

Issues/PRs references

None

[MrKevinWeiss]

Master:

   text	   data	    bss	    dec	    hex	filename
 241512	   1140	 102696	 345348	  54504	/data/riotbuild/riotbase/examples/gnrc_networking/bin/native/gnrc_networking.elf

PR:

   text	   data	    bss	    dec	    hex	filename
 241508	   1140	 102696	 345344	  54500	/data/riotbuild/riotbase/examples/gnrc_networking/bin/native/gnrc_networking.elf

I was able to ping6 the tapbr from native in both cases.

@miri64 Does this make sense?

[benpicco]

TAP_MAC_STACKSIZE is #define TAP_MAC_STACKSIZE (THREAD_STACKSIZE_DEFAULT + DEBUG_EXTRA_STACKSIZE).