usb: bam: Fix KW error for accessing unchecked index

idx was accessed in the variable declaration line before it is checked
at a later point.
Remove the early access to idx.

CRs-Fixed: 629914
Change-Id: I5e574a09bf5c367b708168432c3a646a0f5979af
Signed-off-by: Maya Erez <merez@codeaurora.org>