char: tpm: tpm-buf: Add sanity check fallback in read helpers

Fix Smatch-detected issue:

drivers/char/tpm/tpm-buf.c:208 tpm_buf_read_u8() error:
uninitialized symbol 'value'.
drivers/char/tpm/tpm-buf.c:225 tpm_buf_read_u16() error:
uninitialized symbol 'value'.
drivers/char/tpm/tpm-buf.c:242 tpm_buf_read_u32() error:
uninitialized symbol 'value'.

Zero-initialize the return values in tpm_buf_read_u8(), tpm_buf_read_u16(),
and tpm_buf_read_u32() to guard against uninitialized data in case of a
boundary overflow.

Add defensive initialization ensures the return values are always defined,
preventing undefined behavior if the unexpected happens.

Signed-off-by: Purva Yeshi <[e-mail]>
Reviewed-by: Stefano Garzarella <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>

drivers/char/tpm/tpm-buf.c

···
  */
 u8 tpm_buf_read_u8(struct tpm_buf *buf, off_t *offset)
 {
-	u8 value;
+	u8 value = 0;
 
 	tpm_buf_read(buf, offset, sizeof(value), &value);
 
···
  */
 u16 tpm_buf_read_u16(struct tpm_buf *buf, off_t *offset)
 {
-	u16 value;
+	u16 value = 0;
 
 	tpm_buf_read(buf, offset, sizeof(value), &value);
 
···
  */
 u32 tpm_buf_read_u32(struct tpm_buf *buf, off_t *offset)
 {
-	u32 value;
+	u32 value = 0;
 
 	tpm_buf_read(buf, offset, sizeof(value), &value);