USB: msm72k_udc: Clear ep descriptors upon ep disable

Function driver may use stale descriptors if the descriptors
are not cleared upon endpoint disable.  For example if the gadget
is enumerated with a different speed to the previous enumeration,
function drivers use the old speed descriptors.  Function drivers
also use old interface descriptors when a new composition is
selected.

CRs-fixed: 347400
Change-Id: Ic507529c98d3ab262d86e8b5d316081547124cb9
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>