drivers/nfc/pn544.c: fix min_t warnings

Fix these:

  drivers/nfc/pn544.c: In function 'pn544_read':
  drivers/nfc/pn544.c:356: warning: comparison of distinct pointer types lacks a cast
  drivers/nfc/pn544.c:377: warning: comparison of distinct pointer types lacks a cast
  drivers/nfc/pn544.c: In function 'pn544_write':
  drivers/nfc/pn544.c:463: warning: comparison of distinct pointer types lacks a cast
  drivers/nfc/pn544.c:485: warning: comparison of distinct pointer types lacks a cast

Cc: "Matti J. Aaltonen" <matti.j.aaltonen@nokia.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>