Simplify the definition of a macro
Simplify the definition of a macro

Relying on a PSA_VENDOR macro is not ideal, since the standard doesn't
guarantee this macro exists, but OTOH relying on
MBEDTLS_ECP_DP_xxx_ENABLED was even less ideal, so I believe this is
still an improvement.

Signed-off-by: Manuel Pégourié-Gonnard <4ba0d615aeeda88e422ca8fba28fe2938485facf@arm.com>

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