rxrpc: Fix initial resend timeout

The constant for the initial resend timeout is in milliseconds, but the
variable it's assigned to is in microseconds.  Fix the constant to be in
microseconds.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>