powerpc/inst: Reduce casts in get_user_instr()

Declare __gui_ptr as 'u32 *' instead of casting it at each use to
'unsigned int *' (which is an equivalent type).

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Use u32 * instead of unsigned int *]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2c2123998e05535d08ba03a96ea1eea921d06a86.1621516826.git.christophe.leroy@csgroup.eu