drivers: eth: gmac: conservatively compute the number of descriptors

If a pkt has more frags than the number of TX descriptors, we end up in
a deadlock situation, as the whole packet and thus all the frags have to
be mapped in the descriptors at once. That is why the number of
descriptors is defined as CONFIG_NET_BUF_TX_COUNT + 1.

This wrongly assumes that only TX buffers can be used to send data,
however the packets might also come from the RX buffers, like for
example with ICMPv4.

Therefore define the number of descriptors as the maximum of
CONFIG_NET_BUF_RX_COUNT + 1 and CONFIG_NET_BUF_TX_COUNT + 1. This fixes
a deadlock when CONFIG_NET_BUF_TX_COUNT is much smaller than
CONFIG_NET_BUF_RX_COUNT.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>