ssl_write_server_key_exchange refactor: don't use p in the signing phase
ssl_write_server_key_exchange refactor: don't use p in the signing phase

This is in preparation of further splitting ssl_write_server_key_exchange
into several functions.

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