mfd: stmfx: Fix an endian bug in stmfx_irq_handler()

It's not okay to cast a "u32 *" to "unsigned long *" when you are
doing a for_each_set_bit() loop because that will break on big
endian systems.

Fixes: 386145601b82 ("mfd: stmfx: Uninitialized variable in stmfx_irq_handler()")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>