lib: sbi_misaligned_ldst: Fix handling of C.SWSP and C.SDSP

Unlike C.LWSP/C.LDSP, these encodings can be used with the zero
register, so checking that the rs2 field is non-zero is unnecessary.

Additionally, the previous check was incorrect since it was checking
the immediate field of the instruction instead of the rs2 field.

Signed-off-by: Amanieu d'Antras <amanieu@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>