powerpc/xmon: Fix disassembly since printf changes

The recent change to add printf annotations to xmon inadvertently made
the disassembly output ugly, eg:

  c00000002001e058  7ee00026      mfcr    r23
  c00000002001e05c  fffffffffae101a0      std     r23,416(r1)
  c00000002001e060  fffffffff8230000      std     r1,0(r3)

The problem being that negative 32-bit values are being displayed in
full 64-bits.

The printf conversion was actually correct, we are passing unsigned
long so it should use "lx". But powerpc instructions are only 4 bytes
and the code only reads 4 bytes, so inst should really just be
unsigned int, and that also fixes the printing to look the way we
want:

  c00000002001e058  7ee00026      mfcr    r23
  c00000002001e05c  fae101a0      std     r23,416(r1)
  c00000002001e060  f8230000      std     r1,0(r3)

Fixes: e70d8f55 ("powerpc/xmon: Add __printf annotation to xmon_printf()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>