Fix hardfault in ledstrip driver due to division by zero

[digitalentity]

Fixes #2782

[digitalentity]

@stronnag can you test this as well? Seems to work properly on my OMNIBUSF4

[stronnag]

yes, just building it

[stronnag]

Patched onto current dev ...

# version
# INAV/MATEKF405 1.9.0 Feb 18 2018 / 11:58:53 (a2a58544)

# status
System Uptime: 15 seconds
Current Time: 0000-01-01T00:00:00.000+00:00
Voltage: 0.0V (3S battery - OK)
CPU Clock=168MHz, GYRO=MPU6500, ACC=MPU6500
STM32 system clocks:
  SYSCLK = 168 MHz
  HCLK   = 168 MHz
  PCLK1  = 42 MHz
  PCLK2  = 84 MHz
Sensor status: GYRO=OK, ACC=OK, MAG=NONE, BARO=NONE, RANGEFINDER=NONE, OPFLOW=NONE, GPS=NONE
SD card: Startup failed
Stack size: 6144, Stack address: 0x10010000
I2C Errors: 0, config size: 3494, max available config: 131072
ADC channel usage:
   BATTERY : configured = ADC 1, used = none
      RSSI : configured = ADC 3, used = none
   CURRENT : configured = ADC 2, used = none
  AIRSPEED : configured = none, used = none
System load: 1, cycle time: 1003, PID rate: 997, RX rate: 49, System rate: 9
Arming disabled flags: RX CLI

# feature
Enabled: TELEMETRY LED_STRIP BLACKBOX AIRMODE PWM_OUTPUT_ENABLE 

Confirmed working.