bnxt_en: Remove pointless NULL check for sysctl arg1

Those sysctl handlers have been guaranteed to have non-null softc. No
need for NULL check within sysctl handlers.

No functional change intended.

Reviewed by: markj
Tested by: Daniel Porsch <daniel.porsch@loopia.se>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48495

(cherry picked from commit 747fd2db538a85df84ae6ac1e58494295b4a65ee)