bnxt_en: Support configurable CQE coalescing mode

CQE coalescing mode is the same as the timer reset coalescing mode
on Broadcom devices.  Currently this mode is always enabled if it
is supported by the device.  Restructure the code slightly to support
dynamically changing this mode.

Add a flags field to struct bnxt_coal.  Initially, the CQE flag will
be set for the RX and TX side if the device supports it.  We need to
move bnxt_init_dflt_coal() to set up default coalescing until the
capability is determined.

Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>