[U-Boot] [PATCH] fastboot: Replace literal 32 with PART_NAME_LEN

[Eugeniu Rosca]

Running `md5sum drivers/fastboot/fb_mmc.o` before and after the change
returns the same result, so this is expectedly a non-functional
change.

In addition, `git grep -w 32 --  drivers/fastboot/` reveals no other
occurrences of the magic number, except in below line where it stands
for something unrelated to partition name length:
drivers/fastboot/fb_command.c:    char cmdbuf[32];

Reviewed-by: Eugeniu Rosca <rosca.eugeniu at gmail.com>