boards: esp32: revert HEAP size to integer number

The commit ad63ca28 replaced
default value of 65535 to UINT16_MAX. That change might cause
confusion as it suggests that the HEAP size is limited to UINT16_MAX.

For this particular scenario, 65535 bytes is a resonable amount of
HEAP to allow both Wi-Fi and BT working in coexistance, which could be
increased as per application.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>