u_rmnet_ctrl_qti: Add correct check for validating the port number

This change adds correct check for validating the port number to
be used. The max value of port number defined is NR_QTI_PORTS.
So the permitted range of port number to be used is from zero
to (NR_QTI_PORTS-1). Any value of port number outside this range
(i.e. value greater than and equal to NR_QTI_PORTS) is to be marked
as invalid port.

CRs-Fixed: 562195
Change-Id: I655956fda7071c84a15c3f3d1f637aa6120ff645
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>