m68k: amiga: pcmcia: Replace set but not used variable by READ_ONCE()

When building with W=1:

    arch/m68k/amiga/pcmcia.c: In function ‘pcmcia_reset’:
    arch/m68k/amiga/pcmcia.c:29:23: warning: variable ‘b’ set but not used [-Wunused-but-set-variable]
       29 |         unsigned char b;
	  |                       ^

Fix this by using READ_ONCE(), and 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/1436fa6b329c2212aaf020055afbb97e64b6f039.1694613528.git.geert@linux-m68k.org

arch/m68k/amiga/pcmcia.c

@@ -26,11 +26,10 @@ static unsigned char cfg_byte = GAYLE_CFG_0V|GAYLE_CFG_150NS;
 void pcmcia_reset(void)
 {
 	unsigned long reset_start_time = jiffies;
-	unsigned char b;
 
 	gayle_reset = 0x00;
 	while (time_before(jiffies, reset_start_time + 1*HZ/100));
-	b = gayle_reset;
+	READ_ONCE(gayle_reset);
 }
 EXPORT_SYMBOL(pcmcia_reset);