spi: sh-msiof: Do not redefine STR while compile testing

STR is a well-known stringify macro so it should be avoided in drivers
to avoid warnings like this (MIPS architecture while compile testing):

    drivers/spi/spi-sh-msiof.c:76:0: warning: "STR" redefined
     #define STR 0x40 /* Status Register */
    arch/mips/include/asm/mipsregs.h:30:0: note: this is the location of the previous definition
     #define STR(x) __STR(x)

To maintain consistency between all register names add a SI prefix to
all of them.  This also matches register names in datasheet.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200108194319.3171-1-krzk@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>