USB: serial: sierra driver memory reduction

This patch deals with reducing the memory footprint for sierra driver.
This optimization is aimed for embedded software customers.

Some sierra modems can expose upwards of 7 USB interfaces, each possibly
offering different services. In general, interfaces used for the
exchange of wireless data require much higher throughput, hence require
more memory (i.e. more URBs) than lower performance interfaces. URBs
used for the IN direction are pre-allocated by the driver and this patch
introduces a way to configure the number of IN URBs allocated on a
per-interface basis. Interfaces with lower throughput requirements
receive fewer URBs, thereby reducing the RAM memory consumed by the
driver.

NOTE1: This driver has always pre-allocated URBs for the IN direction.

NOTE2: The number of URBs pre-allocated for the low-performance
interfaces has already been extensively tested in previous versions of
this driver.

We also added the capability to log function calls by adding DEBUG flag.
Please note that  this flag is commented out because this is the default
state
for it.

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>