ethernet: ucc_geth: remove {rx,tx}_glbl_pram_offset from struct ucc_geth_private

These fields are only used within ucc_geth_startup(), so they might as
well be local variables in that function rather than being stashed in
struct ucc_geth_private.

Aside from making that struct a tiny bit smaller, it also shortens
some lines (getting rid of pointless casts while here), and fixes the
problems with using IS_ERR_VALUE() on a u32 as explained in commit
800cd6fb ("soc: fsl: qe: change return type of cpm_muram_alloc()
to s32").

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>