cmd: nvedit: fix efi env -e -i command help

cmd: nvedit: fix efi env -e -i command help

The help string for the `setenv -e' command shows a comma being used as
the separator between address and size for the -i option, which deals
with UEFI Variables contents passed as a buffer in memory.
This is no longer the case since commit 2b3fbcb59f41 ("efi_loader: use
':' as separator for setenv -i") and commit 8f0ac536d493 ("efi: change
'env -e -i' usage syntax"), which changed the separator from a comma to
a colon.
Therefore fix this last bit of the help string accordingly.

While at it, fix the comment of function do_env_set_efi(), which also
mentions a comma as separator.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Maxim Uvarov <maxim.uvarov@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>