ntb_netdev: fix sleep time mismatch

The tx_time should be in usecs (according to the comment above the
variable), but the setting of the timer during the rearming is done in
msecs.  Change it to match the expected units.

Fixes: e74bfeed ("NTB: Add flow control to the ntb_netdev")
Suggested-by: Gerd W. Haeussler <gerd.haeussler@cesys-it.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>
Acked-by: Dave Jiang <dave.jiang@intel.com>