lib: rsa: bring exp_len in line when generating a key_prop

lib: rsa: bring exp_len in line when generating a key_prop

The exponent field of struct key_prop gets allocated an uint64_t,
and the contents are positioned from the back, so an exponent of
"0x01 0x00 0x01" becomes 0x0 0x0 0x0 0x0 0x0 0x1 0x0 0x1"

Right now rsa_gen_key_prop() allocates a uint64_t but sets exp_len
to the size returned from the parser, while on the other hand the
when getting the key from the devicetree exp_len always gets set to
sizeof(uint64_t).

So bring that in line with the established code.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>