sfc: simplify DMA mask setting

Christoph says[1] that dma_set_mask_and_coherent() is smart enough to
 truncate the mask itself if it's too long.  So we can get rid of our
 "lop off one bit and retry" loop in efx_init_io().

[1]: https://www.spinics.net/lists/netdev/msg677266.html



Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>