net: bnxt_ptp: fix compilation error

net: bnxt_ptp: fix compilation error

The Broadcom bnxt_ptp driver does not compile with GCC 11.2.2 when
CONFIG_WERROR is enabled. The following error is generated:

drivers/net/ethernet/broadcom/bnxt/bnxt_ptp.c: In function ‘bnxt_ptp_enable’:
drivers/net/ethernet/broadcom/bnxt/bnxt_ptp.c:400:43: error: array
subscript 255 is above array bounds of ‘struct pps_pin[4]’
[-Werror=array-bounds]
  400 |  ptp->pps_info.pins[pin_id].event = BNXT_PPS_EVENT_EXTERNAL;
      |  ~~~~~~~~~~~~~~~~~~^~~~~~~~
In file included from drivers/net/ethernet/broadcom/bnxt/bnxt_ptp.c:20:
drivers/net/ethernet/broadcom/bnxt/bnxt_ptp.h:75:24: note: while
referencing ‘pins’
   75 |         struct pps_pin pins[BNXT_MAX_TSIO_PINS];
      |                        ^~~~
cc1: all warnings being treated as errors

This is due to the function ptp_find_pin() returning a pin ID of -1 when
a valid pin is not found and this error never being checked.
Change the TSIO_PIN_VALID() function to also check that a pin ID is not
negative and use this macro in bnxt_ptp_enable() to check the result of
the calls to ptp_find_pin() to return an error early for invalid pins.
This fixes the compilation error.

Cc: <stable@vger.kernel.org>
Fixes: 9e518f25802c ("bnxt_en: 1PPS functions to configure TSIO pins")
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20220328062708.207079-1-damien.lemoal@opensource.wdc.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>