igc: Do not enable taprio offload for invalid arguments

Only set adapter->taprio_offload_enable after validating the arguments.
Otherwise, it stays set even if the offload was not enabled.
Since the subsequent code does not get executed in case of invalid
arguments, it will not be read at first.
However, by activating and then deactivating another offload
(e.g. ETF/TX launchtime offload), taprio_offload_enable is read
and erroneously keeps the offload feature of the NIC enabled.

This can be reproduced as follows:

    # TAPRIO offload (flags == 0x2) and negative base-time leading to expected -ERANGE
    sudo tc qdisc replace dev enp1s0 parent root handle 100 stab overhead 24 taprio \
	    num_tc 1 \
	    map 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 \
	    queues 1@0 \
	    base-time -1000 \
	    sched-entry S 01 300000 \
	    flags 0x2

    # IGC_TQAVCTRL is 0x0 as expected (iomem=relaxed for reading register)
    sudo pcimem /sys/bus/pci/devices/0000:01:00.0/resource0 0x3570 w*1

    # Activate ETF offload
    sudo tc qdisc replace dev enp1s0 parent root handle 6666 mqprio \
	    num_tc 3 \
	    map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
	    queues 1@0 1@1 2@2 \
	    hw 0
    sudo tc qdisc add dev enp1s0 parent 6666:1 etf \
	    clockid CLOCK_TAI \
	    delta 500000 \
	    offload

    # IGC_TQAVCTRL is 0x9 as expected
    sudo pcimem /sys/bus/pci/devices/0000:01:00.0/resource0 0x3570 w*1

    # Deactivate ETF offload again
    sudo tc qdisc delete dev enp1s0 parent 6666:1

    # IGC_TQAVCTRL should now be 0x0 again, but is observed as 0x9
    sudo pcimem /sys/bus/pci/devices/0000:01:00.0/resource0 0x3570 w*1

Fixes: e17090eb

 ("igc: allow BaseTime 0 enrollment for Qbv")
Signed-off-by: Florian Kauer <florian.kauer@linutronix.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>