idpf: reduce mbx_task schedule delay to 300us

idpf: reduce mbx_task schedule delay to 300us

During the IDPF init phase, the mailbox runs in poll mode until it is
configured to properly handle interrupts. The previous delay of 300ms is
excessively long for the mailbox polling mechanism, which causes a slow
initialization of ~2s:

echo 0000:06:12.4 > /sys/bus/pci/drivers/idpf/bind

[   52.444239] idpf 0000:06:12.4: enabling device (0000 -> 0002)
[   52.485005] idpf 0000:06:12.4: Device HW Reset initiated
[   54.177181] idpf 0000:06:12.4: PTP init failed, err=-EOPNOTSUPP
[   54.206177] idpf 0000:06:12.4: Minimum RX descriptor support not provided, using the default
[   54.206182] idpf 0000:06:12.4: Minimum TX descriptor support not provided, using the default

Changing the delay to 300us avoids the delays during the initial mailbox
transactions, making the init phase much faster:

[   83.342590] idpf 0000:06:12.4: enabling device (0000 -> 0002)
[   83.384402] idpf 0000:06:12.4: Device HW Reset initiated
[   83.518323] idpf 0000:06:12.4: PTP init failed, err=-EOPNOTSUPP
[   83.547430] idpf 0000:06:12.4: Minimum RX descriptor support not provided, using the default
[   83.547435] idpf 0000:06:12.4: Minimum TX descriptor support not provided, using the default

Fixes: 4930fbf419a7 ("idpf: add core init and interrupt request")
Signed-off-by: Brian Vazquez <brianvv@google.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Samuel Salin <Samuel.salin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>