pps: Use sysfs module parameter to replace PPS_MAX_SOURCES.

[SparkingSpork]

Currently, the PPS_MAX_SOURCES is a hard coded constant. On some systems, the current maximum of 16 is too low. This change converts that logic to use a sysfs parameter instead of a hard-coded value and increases the default value to 32.

Testing:

• Built and installed new kernel on target.
  • Confirmed that new /sysfs/module/pps_core/paramters/pps_max_sources existed and was set to default of 32.
  • Confirmed value could be changed.
  • Confirmed invalid values errored.
  • Confirmed that adding pps_core.pps_max_sources=64 to the boot parameters resulted in a value of 64.
• Ran checkpatch.pl

[SparkingSpork]

V2

• Value is now an unsigned integer.
• Switched to module_param instead of module_param_cb
• Added MODULE_PARM_DESC
• Kept old default maximum of 16

[bstreiff]

Closing in favor of the solution in #131