drm/nv50/pm: signedness bug in nv50_pm_clocks_pre()

calc_mclk() returns zero on success and negative on failure but clk is
a u32.

v2: Martin Peres:
- clk should be an int, not a u32

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>