drivers: modem: fix extra bytes sent by mdm_receiver_send()
drivers: modem: fix extra bytes sent by mdm_receiver_send()

mdm_receiver_send() was sending 1 too many bytes of buf.  This ended
up being the NULL terminator.  Size should be reduced prior to the
while check so that this doesn't happen.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/14001

Signed-off-by: Michael Scott <a17fed27eaa842282862ff7c1b9c8395a26ac320@foundries.io>

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