alsa-mixer: Get rid of a compiler warning.

alsa-mixer: Get rid of a compiler warning.

On 64-bit systems LONG_MAX is greater than the largest possible value of a
uint32_t variable, which caused the compiler to warn about a comparison that is
always false. On 32-bit systems pa_atou() can return a value that will overflow
when assigned to e->volume_limit, which has type long, so the comparison was
necessary.

This dilemma is resolved by using pa_atol() instead of pa_atou().