USB: gadget: function: remove redundant initialization of 'tv_nexus'

Pointer tv_nexus is being initialized a value and this is never read
and is later being updated with the same value. Remove the redundant
initialization so that the assignment to tv_nexus is performed later
and more local to when it is being read.

Cleans up clang warning:
drivers/usb/gadget/function/f_tcm.c:1097:25: warning: Value stored to
'tv_nexus' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>