tsnep: Fix unused warning for 'tsnep_of_match'

Kernel test robot found the following warning:

drivers/net/ethernet/engleder/tsnep_main.c:1254:34: warning:
'tsnep_of_match' defined but not used [-Wunused-const-variable=]

of_match_ptr() compiles into NULL if CONFIG_OF is disabled.
tsnep_of_match exists always so use of of_match_ptr() is useless.
Fix warning by dropping of_match_ptr().

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>