msm: mhi_dev: Update num_reqs for ADB channel

num_reqs feild as part of chan_attr is not updated properly, it is set
to true. So there is only 1 reqs allocated for this channel. This causes
the channel to run of reqs easily causing ADB to stall. Updated num_reqs
to 50.

Change-Id: I1d4ec246c5879dbf1e9ca38ceeec7ffd6b8feccb
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>