bam_dmux: Add workaround for modem crash.

Effectively reverts "msm: bam_dmux: Add locking around platform device
calls" (3688ab952920247bc79a1dd89293bd6449fb1127) for at least xt901,
as this commit is responsible for their modem crash and boot loop.

Kanged from d2 kernel for d2usc

Change-Id: Iafb7092f448f31c0b5a08c20d9d1921f356014a1