Refactor style of the ENA driver

• Change all conditional checks in "if" statement to boolean expressions
• Initialize variables with too complex values outside the declaration
• Fix indentations
• Move code associated with sysctls to ena_sysctl.c file
• For consistency, remove unnecesary "return" from void functions
• Use if_getdrvflags() function instead of accesing variable directly

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12860