[cs89x0] Remove conflicting macro definitions

Remove macros which aren't used anywhere in the driver, and which
conflict with macros of the same name used in the EFI headers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/cs89x0.h

 #define PNP_CNF_DMA 0x74
 #define PNP_CNF_MEM 0x48
 
-#define BIT0 1
-#define BIT15 0x8000
-
 /*
  * Local variables:
  *  c-basic-offset: 8