nfp: don't depend on compiler constant propagation

Matthias reports:

  nfp_eth_set_bit_config() is marked as __always_inline to allow gcc to
  identify the 'mask' parameter as known to be constant at compile time,
  which is required to use the FIELD_GET() macro.

  The forced inlining does the trick for gcc, but for kernel builds with
  clang it results in undefined symbols:

  drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.o: In function
    `__nfp_eth_set_aneg':

drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c:(.text+0x787):
    undefined reference to `__compiletime_assert_492'

drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c:(.text+0x7b1):
    undefined reference to `__compiletime_assert_496'

  These __compiletime_assert_xyx() calls would have been optimized away
if
  the compiler had seen 'mask' as a constant.

Add a macro to extract the mask and shift and pass those to
nfp_eth_set_bit_config() separately.

Reported-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Tested-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>