bnxt_en: Use firmware provided message timeout value.

Before this patch, we used a hardcoded value of 500 msec as the default
value for firmware message response timeout.  For better portability with
future hardware or debug platforms, use the value provided by firmware in
the first response and store it for all susequent messages.  Redefine the
macro HWRM_CMD_TIMEOUT to the stored value.  Since we don't have the
value yet in the first message, use the 500 ms default if the stored value
is zero.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>