Change in flashrom[master]: ni845x_spi: Fix signed - unsigned comparisons

[Angel Pons (Code Review)]

Attention is currently required from: Nico Huber, Miklós Márton, Anastasia Klimchuk.
Angel Pons has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/56637 )

Change subject: ni845x_spi: Fix signed - unsigned comparisons
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Patch Set 1:

(2 comments)

File ni845x_spi.c:

https://review.coreboot.org/c/flashrom/+/56637/comment/9582fb63_3b0f3859 
PS1, Line 556: CS_number = CS_str[0] - '0';
> That's not a problem though, is it? These are just numbers in C. It might […]

Is underflow behavior defined for `unsigned char`?

https://review.coreboot.org/c/flashrom/+/56637/comment/d6659230_f79066b4 
PS1, Line 556: CS_number = CS_str[0] - '0';
             : 		free(CS_str);
             : 		if (strlen(CS_str) > 1 || 7 < CS_number) {
             : 			msg_perr("Only CS 0-7 supported\n");
             : 			return 1;
             : 		}
> I would just use strtoul() TBH :)

It's simpler, sounds good.