sfc: Replace some literal constants with EFX_PAGE_SIZE/EFX_BUF_SIZE

The 'page size' for PCIe DMA, i.e. the alignment of boundaries at
which DMA must be broken, is 4KB.  Name this value as EFX_PAGE_SIZE
and use it in efx_max_tx_len().  Redefine EFX_BUF_SIZE as
EFX_PAGE_SIZE since its value is also a result of that requirement,
and use it in efx_init_special_buffer().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/nic.h

@@ -65,6 +65,11 @@ enum {
 #define FALCON_GMAC_LOOPBACKS			\
 	(1 << LOOPBACK_GMAC)
 
+/* Alignment of PCIe DMA boundaries (4KB) */
+#define EFX_PAGE_SIZE	4096
+/* Size and alignment of buffer table entries (same) */
+#define EFX_BUF_SIZE	EFX_PAGE_SIZE
+
 /**
  * struct falcon_board_type - board operations and type information
  * @id: Board type id, as found in NVRAM