bnxt_en: Mask the bd_cnt field in the TX BD properly

bnxt_en: Mask the bd_cnt field in the TX BD properly

The bd_cnt field in the TX BD specifies the total number of BDs for
the TX packet.  The bd_cnt field has 5 bits and the maximum number
supported is 32 with the value 0.

CONFIG_MAX_SKB_FRAGS can be modified and the total number of SKB
fragments can approach or exceed the maximum supported by the chip.
Add a macro to properly mask the bd_cnt field so that the value 32
will be properly masked and set to 0 in the bd_cnd field.

Without this patch, the out-of-range bd_cnt value will corrupt the
TX BD and may cause TX timeout.

The next patch will check for values exceeding 32.

Fixes: 3948b05950fd ("net: introduce a config option to tweak MAX_SKB_FRAGS")
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250321211639.3812992-2-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>