msm: mhi_dev: Ensure event buffer size reflects the right size

We are setting event buffer size with the requested size and then
allocating the buffer for event requests (ereqs). But if the memory
allocation for ereq buffer fails, the event-buffer-size variable
is not getting set to zero.

Due to this, if a client calls open channel multiple times,
our logic proceeded without really allocating the buffer for ereqs.
And while processing completion events, we are observing null pointer
dereference issues.

Change-Id: I10bee8428d178ef5a7f0fe5fcd93fcce8dcbe27d
Signed-off-by: Veerabhadrarao Badiganti <quic_vbadigan@quicinc.com>