drm/nouveau/clk: fix gcc-7 -Wint-in-bool-context warning

gcc thinks that interpreting a multiplication result as a bool
is confusing:

drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c: In function 'read_pll':
drivers/gpu/drm/nouveau/nvkm/subdev/clk/gt215.c:133:8: error: '*' in boolean context, suggest '&&' instead [-Werror=int-in-bool-context]

Adding a temporary variable to contain the divisor helps make
it clear what is going on and avoids that warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>