m68k: sun3: Fix DEBUG_MMU_EMU build

With DEBUG_MMU_EMU enabled:

    arch/m68k/sun3/mmu_emu.c: In function ‘mmu_emu_handle_fault’:
    arch/m68k/sun3/mmu_emu.c:420:38: error: implicit declaration of function ‘get_fs’; did you mean ‘sget_fc’? [-Werror=implicit-function-declaration]
      420 |         pr_info("seg:%ld crp:%p ->", get_fs().seg, crp);
	  |                                      ^~~~~~

    [...]

    arch/m68k/sun3/mmu_emu.c:420:46: error: request for member ‘seg’ in something not a structure or union
      420 |         pr_info("seg:%ld crp:%p ->", get_fs().seg, crp);
	  |                                              ^

Fix this by reintroducing and using a helper to retrieve the current
value of the DFC register.

While at it, replace "%p" by "%px", as there is no point in printing
obfuscated pointers during debugging.

Fixes: 9fde0348 ("m68k: Remove set_fs()")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/b1d12a1d24b4aea9f98d905383ba932b2dc382e6.1737387419.git.geert@linux-m68k.org