target/riscv: re-apply patch do stop avoid warnings when a non-existent CSR is hidden

[aap-sc]

the original fix was introduced in b201a5d but was lost in 3883b03

[aap-sc]

@MarekVCodasip , @en-sc I'm going to wait till Friday and if no objections - I'll proceed with merging this. We already had this patch reviewed - it was occasionally lost in 3883b03

[MarekVCodasip]

LGTM

[en-sc]

@aap-sc, this patch is not the same as b201a5d -- it changes the warning in riscv_reg_impl_expose_csrs() instead of the one in riscv_reg_impl_hide_csrs(). I'd like to revert it, since the commit message is extremely misleading.

The original issue (the one that b201a5d addressed) is still present in the current version:

riscv-openocd/src/target/riscv/riscv_reg.c

Lines 758 to 763 in c85d4e1

	if (!reg->exist) {
	LOG_TARGET_WARNING(target,
	"Not hiding CSR %d: register does not exist.",
	csr_number);
	continue;
	}