bnxt_en: fix error handling when flashing from file

After bnxt_hwrm_do_send_message() was updated to return standard error
codes in a recent commit, a regression in bnxt_flash_package_from_file()
was introduced.  The return value does not properly reflect all
possible firmware errors when calling firmware to flash the package.

Fix it by consolidating all errors in one local variable rc instead
of having 2 variables for different errors.

Fixes: d4f1420d

 ("bnxt_en: Convert error code in firmware message response to standard code.")
Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>