tsnep: Fix NAPI scheduling

According to the NAPI documentation networking/napi.rst, drivers which
have to mask interrupts explicitly should use the napi_schedule_prep()
and __napi_schedule() calls.

No problem seen so far with current implementation. Nevertheless, let's
align the implementation with documentation.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>