iflib: Make the deferral test in iflib_txd_db_check() an early return

[rcm]

Summary

Invert the test so the doorbell write is no longer nested inside the
conditional, and wrap its comments to 80 columns. Fix a typo in one of
them.

No functional change intended.

Sponsored by: Rubicon Communications, LLC ("Netgate")

Test Plan

No functional change intended.

[kgalazka]

sys/net/iflib.c

3476

nit: extra space

[rcm]

removed extraneous space