audio/msm7x30: perform null checks when calling libhtc_acoustic functions

Fixes null pointer dereference when msm72xx_set_audpre_params and
msm72xx_enable_audpre are called from libhtc_acoustic.so that does
not have those functions (such as those for msm8x60 platform).

Change-Id: Ifa4357835e938b374e09908c34c14c8c5c52ce10