config.h: Don't use arithmetical exp for SSL_DTLS_MAX_BUFFERING
config.h: Don't use arithmetical exp for SSL_DTLS_MAX_BUFFERING

The functions requires_config_value_at_least and requires_config_value_at_most
only work with numerical constants.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls