pwm: tegra: Avoid potential overflow for short periods

For very short periods, the result of the division might overflow the
unsigned long hz variable (on 32-bit architectures). Avoid that by
making it an unsigned long long. While at it, also remove an unneeded
local variable whose only purpose is to store a temporary computation.

Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>