tua6100: Avoid build warnings.

Rename _P to _P_VAL and _R to _R_VAL to avoid global
namespace conflicts:

drivers/media/dvb-frontends/tua6100.c: In function ‘tua6100_set_params’:
drivers/media/dvb-frontends/tua6100.c:79: warning: "_P" redefined
 #define _P 32

In file included from ./include/acpi/platform/aclinux.h:54,
                 from ./include/acpi/platform/acenv.h:152,
                 from ./include/acpi/acpi.h:22,
                 from ./include/linux/acpi.h:34,
                 from ./include/linux/i2c.h:17,
                 from drivers/media/dvb-frontends/tua6100.h:30,
                 from drivers/media/dvb-frontends/tua6100.c:32:
./include/linux/ctype.h:14: note: this is the location of the previous definition
 #define _P 0x10 /* punct */

Signed-off-by: David S. Miller <davem@davemloft.net>