usb: f_gsi: Clean up low memory buffer size/depth definitions

Move the #ifdef CONFIG_USB_LOW_MEM_VARIANT conditionals from
to the f_gsi.h header and simplify the usages in the code to
use a single set of #defines.

Also add a redefinition of the ADPL buffer depth to 15 in
case of low memory variant.

Change-Id: I138ec69effd5ef38a6de95b8ce6a1f356d6079bb
Signed-off-by: Jack Pham <jackp@codeaurora.org>