STM32F756xG: fix IAR RAM size check

Author: 0xc0170

targets/TARGET_STM/TARGET_STM32F7/TARGET_STM32F756xG/device/TOOLCHAIN_IAR/stm32f756xg.icf

@@ -26,7 +26,7 @@ ITCM-RAM:  16K (0x3FFF)   @0x00000000
 if (!isdefinedsymbol(MBED_ROM_START)) { define symbol MBED_ROM_START  = 0x8000000; }
 if (!isdefinedsymbol(MBED_ROM_SIZE)) { define symbol MBED_ROM_SIZE   = 0x200000; }
 if (!isdefinedsymbol(MBED_RAM_START)) { define symbol MBED_RAM_START  = 0x20010000; }
-if (!isdefinedsymbol(MBED_RAM_START)) { define symbol MBED_RAM_SIZE   = 0x40000; }
+if (!isdefinedsymbol(MBED_RAM_SIZE)) { define symbol MBED_RAM_SIZE   = 0x40000; }
 if (!isdefinedsymbol(MBED_RAM1_START)) { define symbol MBED_RAM1_START = 0x20000000; }
 if (!isdefinedsymbol(MBED_RAM1_SIZE)) { define symbol MBED_RAM1_SIZE  = 0x10000; }