sfc: siena: Have a unique wrapper ifndef for efx channels header

sfc: siena: Have a unique wrapper ifndef for efx channels header

Both sfc/efx_channels.h and sfc/siena/efx_channels.h used the same
wrapper #ifndef EFX_CHANNELS_H, this patch changes the siena define to be
EFX_SIENA_CHANNELS_H to avoid build system confusion.

This fixes the following build break:
drivers/net/ethernet/sfc/ptp.c:2191:28:
error: ‘efx_copy_channel’ undeclared here (not in a function); did you mean ‘efx_ptp_channel’?
  2191 |  .copy                   = efx_copy_channel,

Fixes: 6e173d3b4af9 ("sfc: Copy shared files needed for Siena (part 1)")
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Cc: Edward Cree <ecree.xilinx@gmail.com>
Acked-by: Martin Habets <habetsm.xilinx@gmail.com>
Link: https://lore.kernel.org/r/20220518065820.131611-1-saeed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>