platform-drivers: msm: sps: fix sleeping function called from atomic issue

sps_disconnect() calls mutex inside spinlock protected code, and thus gets
the warning "sleeping function called from invalid context". Change
spinlock to mutex in sps_disconnect().

Change-Id: Ied30595f20d0f5976d53575138b072ba251fc10e
Signed-off-by: Yan He <yanhe@codeaurora.org>