psp: Expand PSP acronym in INET_PSP help description

People not very intimate with PSP may not know the meaning of this
recursive acronym.  Hence replace the half-explanatory "PSP protocol" in
the help description by the full expansion, like is done in the linked
PSP Architecture Specification document.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/ae13c3ed7f80e604b8ae1561437a67b73549e599.1758784164.git.geert+renesas@glider.be


Signed-off-by: Jakub Kicinski <kuba@kernel.org>