applications: serial_lte_modem: Remove Kconfig from workaround

Zephyr compliance checks won't accept missing Kconfigs
so use the previously used value as a hard coded until
new code is merged.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

applications/serial_lte_modem/src/slm_cmux.c

@@ -33,7 +33,7 @@ LOG_MODULE_REGISTER(slm_cmux, CONFIG_SLM_LOG_LEVEL);
 
 /* Workaround before upmerge */
 #ifndef MODEM_CMUX_WORK_BUFFER_SIZE
-#define MODEM_CMUX_WORK_BUFFER_SIZE CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE
+#define MODEM_CMUX_WORK_BUFFER_SIZE 536
 #endif
 
 static struct {