mac802154: Increase tx_buffer_len

Increase the buffer length from 10 to 300 packets. Consider that traffic on
mac802154 devices will often be 6LoWPAN, and a full-length (1280 octet)
IPv6 packet will fragment into 15 6LoWPAN fragments (because the MTU of
IEEE 802.15.4 is 127).  A 300-packet queue is really 20 full-length IPv6
packets.

With a queue length of 10, an entire IPv6 packet was unable to get queued
at one time, causing fragments to be dropped, and making reassembly
impossible.

Signed-off-by: Alan Ott <alan@signal11.us>
Signed-off-by: David S. Miller <davem@davemloft.net>