mwifiex: remove max_tx_buf_size

max_tx_buf_size is not used any more after reconfiguration of
tx buffer size has been removed.

Also add missing curr_tx_buf_size update while dumping debug info
via debugfs.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>