drivers: modem: gsm: set AT+IFC if hw-flow-control is enabled

If hw-flow-control is set in the uart section of the device tree source,
it is read by gsm_ppp.c and AT+IFC=2,2 is sent to the modem.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/46928

.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>