cxgb4: Wait for device to get ready before reading any register

Call t4_wait_dev_ready() before attempting to read the PL_WHOAMI register
(to determine which function we have been attached to). This prevents us from
failing on that read if it comes right after a RESET.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>