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

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