r8169: remove nr_frags argument from rtl_tx_slots_avail

The only time when nr_frags isn't SKB_MAX_FRAGS is when entering
rtl8169_start_xmit(). However we can use SKB_MAX_FRAGS also here
because when queue isn't stopped there should always be room for
MAX_SKB_FRAGS + 1 descriptors.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/3d1f2ad7-31d5-2cac-4f4a-394f8a3cab63@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>