cxl/test: Simplify fw_buf_checksum_show()

cxl/test: Simplify fw_buf_checksum_show()

First, just use sha256() instead of a sequence of sha256_init(),
sha256_update(), and sha256_final().  The result is the same.

Second, use *phN instead of open-coding the conversion of bytes to hex.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160645.3198-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>