logger: replace hardcoded constant in rtt backend
logger: replace hardcoded constant in rtt backend

Replace hardcoded report limit of dropped message by macro.

Signed-off-by: Pavel Kral <dbcc8f8289ea7748ef3e2d49ba0fc4b3403d93d2@omsquare.com>

nashif/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,punitvara/zephyr,explora26/zephyr,punitvara/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr