rp2/CMakeLists.txt: Fix flash size check logic.

Follow up to 6bfb83e30a, if the variable
`PICO_FLASH_SIZE_BYTES` is not a numeric constant, eg "(2 * 1024 * 1024)",
then it won't pass the GREATER check.  So change the if logic to just test
if it's defined.

Signed-off-by: Dryw Wade <dryw.wade@sparkfun.com>

ports/rp2/CMakeLists.txt

@@ -528,7 +528,7 @@ target_link_options(${MICROPY_TARGET} PRIVATE
     -Wl,--wrap=runtime_init_clocks
 )
 
-if(PICO_FLASH_SIZE_BYTES GREATER 0)
+if(DEFINED PICO_FLASH_SIZE_BYTES)
     target_link_options(${MICROPY_TARGET} PRIVATE
         -Wl,--defsym=__micropy_flash_size__=${PICO_FLASH_SIZE_BYTES}
     )