net: mvpp2: remove mvpp2_txq_pend_desc_num_get() function

The mvpp2_txq_pend_desc_num_get() function only selects a TX queue, and
reads the number of pending descriptors. It is used in only one place,
in mvpp2_txq_clean(), where the TX queue has already been selected by a
write to MVPP2_TXQ_NUM_REG.

Therefore, this function is useless, and the caller can simply read the
value of the MVPP2_TXQ_PENDING_REG register instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>