m68k: sun3: Remove unused vsprintf() return value in prom_printf()

When building with W=1:

    arch/m68k/sun3/prom/printf.c: In function ‘prom_printf’:
    arch/m68k/sun3/prom/printf.c:28:13: warning: variable ‘i’ set but not used [-Wunused-but-set-variable]
       28 |         int i;
	  |             ^

As the return value of vsprintf() is unused, and serves no practical
purpose here, fix this by removing the variable.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/94a1e61b1651ff05f4a59655d9b8c1ac5338f60c.1694613528.git.geert@linux-m68k.org