usb: musb: dsps: use msecs_to_jiffies instead

when polling, we were using n * HZ (where n is
an integer in seconds), however HZ isn't always
correct if we're using cpufreq. A better way
is to use msecs_to_jiffies(n) (where n is now
an integer in miliseconds).

while at that, also rename poll_seconds to poll_timeout
and  change its type to unsigned int.

Signed-off-by: Felipe Balbi <balbi@ti.com>