hw/intc/sh_intc: Turn some defines into an enum

hw/intc/sh_intc: Turn some defines into an enum

Turn the INTC_MODE defines into an enum and clean up the function
returning these to make it clearer by removing nested ifs and
superfluous parenthesis. The one remaining #define is a flag which is
moved further apart by changing its value from 8 to 0x80 to leave some
spare bits as this is or-ed with the enum value at some places.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <4adf4e1ac9d2e728e5a536c69e310d77f0c4455a.1635541329.git.balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>